[sempervivens]

Recently I was offered this, which I couldn't refuse:

A rare first series cal. 135, all original, from its first owner!

A Zenith cal. 135 as such is not rare, but a first series is - the more so in all original condition.

This nicely aged, original dial, should make all other dials blush, as they look repainted in comparison (many are).

The solid but elegant steel case is typical for a cal. 135: the same steel case was still used for later editions.

The case looks unpolished, with chamfered edges on the lugs.

it is a case in three parts.

Caseback looks unpolished as well.

The crown has the correct logo for the period, although I think it was replaced at some point during service (possibly in 1963), as it doesn't look a perfect fit.

Original buckle:

This early cal. 135 has chromium plated hands and dial markers. What a contrast with later, more luxurious editions of cal. 135, when half of all cal. 135's were fitted with gold cases!

The lettering is also different from later editions. The emphasis here is on the 'chronometre', which has a thicker font than the 'Zenith' name, whereas in later examples the Zenith name appears bigger than the 'chronometre'.

But all its beauty is on the inside ...

As you can see in this picture the balance is uncut, monometallic, whereas the original balance for a first series cal. 135 was a bimetallic, split balance : I'll explain more about this in the second part of this post. I conclude that the balance was replaced during service with a second series caliber 135 monometallic balance.

Caseback shows only one service date (1963).

Although I'm not worthy of it, I'm of course happy and grateful that I got this: a caliber 135 should be the cornerstone of any collection.

Hope you enjoyed this.

 

[chicagozqian]

glad for you.
expecting the 2nd post

 

[sempervivens]

Part II: Zenith caliber 135 : first, second and third series

There are several differences between the first and second series cal. 135.

According to Rössler, Zenith could not find any documentation about these differences.

Fortunately Nicola1960 made it simple and clear for everybody to see in this post on orologi & passioni : Zenith Chronometre cal. 135 nuovo arrivo - page 2

Here is a synthesis (with some additions) of what I found:

The first series (1949-1952) has a goldplated movement with "Côtes de Geneve" finish; it has a bimetallic, split, screwed balance;
it can especially be recognised by a "pin with round head" on the main plate near the crown wheel, which can be seen in this picture:

The serial numbers for the first series are in the range 3869xxx/3870xxx and 4062xxx (see also below).

The second series (1953-1960) has a rhodium plated movement (*) with "Côtes de Geneve"; a monometallic, uncut, screwed balance;
the movement is marginally simplified, which can be noted visually by the absence of the "pin with rounded head" (replaced by a screw, not visible on the main plate);
(*) a few second series movements were still goldplated, but can easily be recognised as second series by the absence of the pinhead under the crown wheel.

The serial numbers for the second series are in the range 4217xxx until 4872xxx (see also below).

The third series (1962) is the same as the second series, but the finishing of the decoration is less refined. In the third series there were also movements without the mention 'chronometre'.

The serial numbers for the third series start in the range 5362xxx and end in the range 6326xxx (see also below).

See also these 3 pictures by Nicola1960, showing the differences between the first, second and third series:

First series: goldplated; split bimetallic balance wheel; round pinhead near crown wheel

Second series: rhodium plated; uncut monometallic balance wheel; pinhead near crown wheel is absent

Third series: same as second series, but less anglage in the finishing, and in this example the 'chronometre' is missing

Actual production of cal 135 started in 1949 and ended in 1962. Production thus was for 13 years : 1949-1962.

Note also that it is a 13 lignes movement. There definitely is something special about that number 13 :think:!

With a total of 11000 units made, cal. 135 is not a rare watch. Still, it has become relatively rare, because it is much in demand by collectors.

To complete this overview I made a list of serial numbers of the three series, which allows to discern the different production batches during the years.

As a general conclusion one can say that the first series cal. 135 appears especially rare. Most cal. 135's belong to the second series, especially around 1954 many were made (after caliber 135 had already won the chronometer competition for several years in a row).

But you can draw many conclusions, I will let the results speak for themselves:

There is one example which could represent the oldest production run of cal. 135:
it has no movement number, a repainted dial, and a steel case # 8763119. Judging by the case number, it was produced around 1949.

Judging by the case numbers, this was followed around 120000 watches later (ca. 1-2 years later) by a second production run of first series 135's, of which I found 4 examples, belonging to one batch:

3869877 (steel case) (ca. 1950)
3869905 (steel case 8884032)
3869940 (steel case 8884007)
3869965 (gold case 456208)
3870001 (gold case)
3870041 (gold case 456294) : "produced on 23.10.1951"
3870073 (gold case 488228) with engraving: Aarburg 8 Feb 1957
3870187 (gold case 534723)
3870210 (gold case 527936)

Now following the movement numbers, around 190000 watches later (= 1-2 years later) there came a third production run of the first series cal. 135, of which I found two examples:

4062567 (steel case 8990993) Favre Leuba Zenith; with inscription SANTRAM 17th May 1954
4062593 (steel case; ca. 1952)
4062647 (steel case)
4062796 (this additional example was reported by Hans61)

Conclusion concerning the first series of cal. 135:

I found only seven examples of the first series cal. 135, which were produced in three separate production runs, dating to 1949-1950/51-1952.
(later updated to fourteen examples)

Around 150000 watches later (ca. 1,5 years later) there followed the second series of caliber 135.

There have been at least seven production runs of the second series, starting in 1953/1954, and continuing with regular intervals until 1960. Additionally, there are some observatory chronometers.

4217256 (gold case) (ca. 1953/1954)
4217359 (gold case)
4217397 (gold case)
4217481 (steel case 8991151)
4217431 (steel case 8991171)
4217xxx (steel case 8991198)
4217646 (steel case, number worn to illegibility)
4217677 (steel case 8991227) second series movement, but goldplated instead of rhodinated
4217759 (gold case was probably melted) with goldplated finish instead of rhodinated
4217824 (rose gold): produced 25.02.1954
4217967 (steel case 9006110)

4250320 (steel case 9048110)
4250423 (steel case 90xxxxx)
4250xxx (gold case): produced 12.11.1954
4250520 (steel case 9092160)
4250589 (steel case)
4250628 (steel)
4250632 (gold case 582162)
4250637 (steel case, SN 744D777)
4250709 movement and dial (gold case probably was melted)
4250728 (gold)
4250746 (gold case probably was melted)
4250791 dial with hollow star and 'importé de suisse' (steel case 9175164)
42508xx (gold case 542326) (ebay it 6000 £)
4250826 (rose gold case 582162) 12.10.1954
4250828 (gold case 542128) with engraving 1956
4251060 (9 K gold case 28638 with Zenith London's subsidiary's Hallmark "ZWCo") 19.11.1954 (ebay.uk 5200 £)
4251122 (steel)
4251186 (rose gold case 542386)
4251256 (gold case 542166)
4251282
4251289 (9 K gold case 28762)

4405086 (rose gold case 612336) produced 09.09.1955
4405102 (steel case)
4405127 (steel)
4405162 (gold case 612292)
4405182 (rose gold)
4405296 (steel)
4405427
4405485 (gold)
4405486 (gold case)
4405497 (steel 9254940)
4405594 (gold case 612384)
4405600 (gold case melted?)
4405604 (gold case 617364)
4405647 (steel case 9254827)
4405990 (gold case 648510)

(ca. 1956/57):
4518048 (steel case 9254958)
4518072 Favre-Leuba Zenith
4518074 Favre-Leuba Zenith (steel case 9306384) (sold in auction on 4 May 2013 for GBP 3.600)
4518260 (steel case 9250170)
4518334 (gold case 648524)
4518391
4518393 (steel case 919449x)
4518450 (steel case 9194461)
4518564
4518584 Favre-Leuba Zenith
4518650
4518695 (steel case 9306425)
4518770 Port Royal (gold case 687760)
4518832
4518908 (steel case 9255048) ebay brazil 4400 $ (Oct 2016)
4519393 (goldplated movement but no côtes de geneve?); Favre-Leuba Zenith

4641318 observatory chronometer

4663046 (steel case 9297436)
466306x (steel case 92773x3) 17.07.1956
4663081 (gold case 677891)
4663146 (9 K gold case 30817)
4663224 (steel case 9297324)
4663289
4663330
4663353 Port Royal
4663400 Port Royal (gold case 772852)
4663401
4663438
4663439 (gold case)
4663xxx (gold case) produced 25.04.1957
4663523
4663687Port Royal (gold case 687877) (€ 5800)
4663727 (steel case) (this was reported stolen, from Gombrich)
4663824
4663828
4663920
4663952
4663983 (steel case)

(ca. 1959
4743020
4743033 Port Royal
4743050 (steel case 9415400)
4743106 (gold case 765526) produced 30.10.1959 (Port Royal I model)
4743115 (steel case 9483935) Port Royal
4743131 (gold case)
4743195 (steel case 9385925) produced 13.05.1959

4732508 observatory chronometer
4732528 observatory chronometer with observatory certificate dated to 1960
4732661 observatory chronometer
4732666 observatory chronometer

4743235 (gold capped case 9529985)
4743290 (gold capped case 9530030)
4743387 Port Royal (steel case 9483912) (€ 14.995, Veldeszenith)
4743392 Port Royal (gold case 765562) produced 1959
4743437 (steel case)
4743475 Zenith 2000 (steel case 9485375) engraved 26.3.1960
4743679 Port Royal (steel case 9493959)
4743730 Port Royal (gold case 816530)
4743763
4743803 Port Royal
4743921
4743926 (steel case 9579836)

(ca. 1960
4872096 Favre-Leuba Zenith (steel case 9564716) 7.9.1960
4872109
4872199 (steel case 9564729) (ebay.uk £ 4000)
4872349 Port Royal
4872469 Zenith 2000 (steel case 9579919)
4872480
4872674 (steel case)
4872718 Port Royal (steel case 9651915)
4872857 (steel case 9663247)
4872882
4872943 Port Royal (steel case 9651873)


Conclusion: the second series is the most prolific and was made from ca. 1953 until 1960, with a peak in production around 1954.

Finally, the third series (ca. 1962-69) :
according to Rössler the last production year of cal. 135 was 1962, the remaining movements of which were then used up until 1969.

Still there are different serial numbers for different years (between 1962 and 1967):

5362010 Zenith 2000 steel case between lugs 9668880 engraved 11.8.1962
5362084 Zenith 2000 steel case engraved 19.11.62
5362134 non-chronometer Zenith 2000
5362145 Zenith 2000 'produced 1959'
5362178 Zenith 2000
5362197 non-chronometer Zenith 2000
5362590 Zenith 2000
5362625 Port Royal (steel case 9697655)
5362627 non-chronometer Zenith 2000 (serial number under balance)
5362666 Port Royal (steel case 9697666)
5362680 Port Royal (steel case 9697590)

(ca. 1963
5414172 Zenith 2000
5414383
5414499 Zenith 2000 (on back) case 779A182 (between lugs) (case number dates ca. 1967)
5414998 Zenith 2000 (on back) case 779A593 (between lugs)
5414531 Zenith 2000 (steel case 9753571 between lugs)
5414695 Zenith 2000 (steel case with engraving 30-5-1964)

(ca. 1964/67
Zenith 2000 (dial and movement signed chronometre) steel case 284A778 with engraving 5th Sept 1966
5816755 Zenith 2000 (dial and movement signed chronometre) steel case 284A821
(no serial number visible) Zenith 2000 (steel case 429Axxx between lugs) (case number dates ca. 1965) sold Feb 19, 2013 US $7,700.00 (Italy)
5414998 Zenith 2000 (steel case, number 799A593 between the lugs) dated '1963' (but case number dates ca. 1966/67)
5455034

58xxxxx Zenith 2000 non-chronometer (mentions "2000" only on the back)

63xxxxx chronometer Zenith 2000 (ca. 1966-67)
6326611 non-chronometer Zenith 2000
632xxxx Favre-Leuba Zenith chronometer with Zenith 2000 caseback
6326479 Zenith 2000 (case 799A648) (case number dates ca. 1967)
6326611 Zenith 2000
6326627 Zenith 2000

Conclusion concerning the third series: most common model here is the Zenith 2000, but in the first batch we still see some Port-Royal's. Often the serial numbers of the movements are not visible in pictures, because ca. 1963 Zenith started to engrave the numbers below the balance. Additionally, the Zenith 2000 had a special caseback (with Zenith 2000 logo) : these have a case number between the lugs. Since the movements for the third series were supposedly all made in 1962, I'll mention Nicola's theory that the serial numbers were added only when the watches were finished.

I hope you like this and may find it useful.

Thanks to all who have contributed pics or information.

Special thanks to Nicola1960 for explaining the differences between the three series and providing so much useful information.

Also thanks to Hans61 for adding many more serial numbers to the list. Danke!

To conclude with some catalog illustrations.

Thanks to Nicola1960 again for the following three pictures from a 1953 catalog:

Thanks to LouS for the following pic from a catalog (ca. 1954):

And thanks to all...

 

[chicagozqian]

very interesting thanks

 

[Hartmut Richter]

Wonderful watch - congratulations!

I am not sure where you got the production of 13500 instead of 11000 from, there seems to be no documentation (Rössler also lists 11000 and, although he not always correct as we know, we have no proof to the contrary here).

The "pin with a rounded head" is a decoupling device to take the hour and minute hands out of the main power transmission while the time is being set, allowing the main movement to run without any extra friction and maintain chronometric accuracy to the greatest possible extent. I have a German article on that (in bad photocopies), if necessary, I will upload the page with the relevant diagramme.

The 13''' size is significant only to the extent that it was the maximum size allowed for movements in the observatory competitions for the wrist watch grade (30mm diameter or 707mm² total area for form movements, which is where the Zenith Cal. 707 - the "potato"! - got its designation from). This is the reason why there are so many 13''' and very few 14''' and 15''' movements around (except chronographs, where every little bit of extra area for the chrono mechanism made a difference - chronographs were almost never tested in observatory competitions anyway!). Any movement larger than 13''' would have to be entered in the pocket watch grade and would then have to compete against movement up to 50mm diameter. Since larger movements are more accurate, all other things being equal, the chances of doing well in that category for a 14''' movement were slim.

Enjoy the watch!

Hartmut Richter

 

[MMMD]

Congratulations on a wonderful addition to your collection!
Well I'll be darned... never noticed that "decoupling" device. My 135 has a gilt movement, a 421xxxx serial number, and no decoupling device.

I had considered it a late first-series movement, but because of this silly little knob near the crown wheel I'll have to call it a transitional movement between the gilt first series and rhodinated second series. There are others like it, as John Chris will attest. Thanks for the information! Looks like there is a sudden cavity in my collection that must be filled.

 

[sempervivens]

Wonderful watch - congratulations!

I am not sure where you got the production of 13500 instead of 11000 from, there seems to be no documentation (Rössler also lists 11000 and, although he not always correct as we know, we have no proof to the contrary here).

The "pin with a rounded head" is a decoupling device to take the hour and minute hands out of the main power transmission while the time is being set, allowing the main movement to run without any extra friction and maintain chronometric accuracy to the greatest possible extent. I have a German article on that (in bad photocopies), if necessary, I will upload the page with the relevant diagramme.

The 13''' size is significant only to the extent that it was the maximum size allowed for movements in the observatory competitions for the wrist watch grade (30mm diameter or 707mm² total area for form movements, which is where the Zenith Cal. 707 - the "potato"! - got its designation from). This is the reason why there are so many 13''' and very few 14''' and 15''' movements around (except chronographs, where every little bit of extra area for the chrono mechanism made a difference - chronographs were almost never tested in observatory competitions anyway!). Any movement larger than 13''' would have to be entered in the pocket watch grade and would then have to compete against movement up to 50mm diameter. Since larger movements are more accurate, all other things being equal, the chances of doing well in that category for a 14''' movement were slim.

Enjoy the watch!

Hartmut Richter

Thanks Hartmut. In the English translation of Rössler on p. 123 he writes 'The movement was produced until 1956 for a total of 13500 units.' Is it the same in the German edition?
I see now that in his movement list on p. 31 he mentions 11000. If he contradicts himself, I would choose the higher number to be on the safe side.
13500 units produced of caliber 135 in 13 years. It's too good, not to be true b-)

 

[sempervivens]

Congratulations on a wonderful addition to your collection!
Well I'll be darned... never noticed that "decoupling" device. My 135 has a gilt movement, a 421xxxx serial number, and no decoupling device.

I had considered it a late first-series movement, but because of this silly little knob near the crown wheel I'll have to call it a transitional movement between the gilt first series and rhodinated second series. There are others like it, as John Chris will attest. Thanks for the information! Looks like there is a sudden cavity in my collection that must be filled.

Thanks M. Well you certainly have a beautiful pair of 135's already, one steel and one gold, I don't see any cavities there ;-)

 

[sempervivens]

thank you

very interesting thanks

 

[D N Ravenna]

Very nice, thanks for sharing!

:-!

Dan

 

[nicola1960]

:-!




P.S.
Aggiungo:
L'orologio Zenith "2000": l'ultimo con il cal. 135

 

[sempervivens]

Very nice, thanks for sharing!

:-!

Dan

Thank You Dan

 

[sempervivens]

:-!




P.S.
Aggiungo:
L'orologio Zenith "2000": l'ultimo con il cal. 135

Wonderful Nicola. Thank You.

Excellent new post you did on the "Zenith 2000" !

PS For your research concerning the "Zenith 2000", note that some have a serial number between the lugs, e.g. 429Axxx.
(Da notare per la tua ricerca "Zenith 2000": alcuni hanno un numero seriale tra le anse, per esempio 429Axxx)

 

[nicola1960]

;-)

 

[sempervivens]

"La perfection existe, Zenith le prouve!" :-! "Il est toujours l'heure Zenith" :-! Pretty cool slogans b-) Nice drawing of the men behind microscopes

 

[Hans61]

Hi sempervivens.
I have added your numbers to my list:

3869877 Cal 135
3869905 Cal 135 Zenith Chronometre 8884032
3869989 Cal 135 Zenith Chronometre 456252 18K
3870001 Cal 135 Zenith Chronometre
3870041 Cal 135 vom 23.10.1951 Zenith 456294
4062593 Cal 135
4062647 Cal 135 vom 26.12.1952 Zenith Chronometre
4062796 Cal 135
4217256 Cal 135 Zenith Chronometre
4217359 Cal 135 Zenith Chronometer
4217397 Cal 135 von 1960 Zenith Chronometre 18K RösslerS126
4217481 Cal 135 Zenith Chronometre 8991151
4217677 Cal 135 Zenith Chronometre 8991227
4217759 Cal 135
4217824 Cal 135 vom 25.02.1954 Zenith Chronometre
4217967 Cal 135 Zenith Chronometre 9006110
4250xxx Cal 135 Zenith Chronometre
4250320 Cal 135 Zenith Chronometre 9048110
4250423 Cal 135 Zenith Chronometre
4250520 Cal 135 Zenith Chronometer 9092160
4250589 Cal 135 Zenith Chronometre
4250628 Cal 135
4250632 Cal 135
4250728 Cal 135 Zenith Chronometre
4250746 Cal 135
4250813 Cal 135 Zenith ???
4251122 Cal 135 Zenith Chronometre
4251282 Cal 135
4251289 Cal 135 Zenith Chronometre 28762 9K
4260375 Cal 135 Zenith Chronometre
4405086 Cal 135 vom 09.09.1955 Zenith Chronometre 612336 18K
4405102 Cal 135 vom 07.06.1955 Zenith Chronometre
4405127 Cal 135 Zenith Chronometre 9175197
4405296 Cal 135 vom 29.04.1955 Zenith Chronometre
4405427 Cal 135 Zenith Chronometre
4405485 Cal 135 Zenith Chronometre 18K
4405486 Cal 135 Zenith Chronometre 542196 18K
4405594 Cal 135 vom 09.09.1955 Zenith Chronometre 612384 18K
4405600 Cal 135 nur Werk
4405647 Cal 135 Zenith Chronometre 9254827
4518074 Cal 135 Favre-Leuba Zenith 9306384
4518391 Cal 135 Zenith Chronometer
4518393 Cal 135 Zenith Chronometre 919449
4518564 Cal 135 Zenith HAU
4518584 Cal 135 von 1955 Zenith Favre-Leuba Chronometer Rössler
4518650 Cal 135 Zenith Chronometre
4518695 Cal 135 Zenith Chronometre 9306425
4518779 Cal 135 Zenith Chronometre
4519393 Cal 135
4641318 Cal 135
4663081 Cal 135 Zenith Chronometre 677891 18K
4663146 Cal 135 Zenith Chronometre 30817 9K
4663289 Cal 135 Zenith Chronometre
4663330 Cal 135 Zenith 1050654
4663353 Cal 135 Zenith Chronometre Port Royal 18K
4663401 Cal 135 Zenith Chronometre
4663438 Cal 135 Zenith HAU
4663439 Cal 135 Zenith
4663710 Cal 135 Zenith Chronometre
4663727 Cal 135 Zenith Chronometre
4663824 Cal 135 Zenith Chronometre
4663828 Cal 135 Zenith Chronometre
4663920 Cal 135 Zenith Chronometre
4663952 Cal 135 Zenith Chronometre
4663983 Cal 135 Zenith Chronometre
4732528 Cal 135-O Zenith Chronometre Spezial Observatoriumsuhr
4732661 Cal 135-O Zenith Chronometre Observatoriumsuhr
4732666 Cal 135-O Ebauche 1950 Zenith Chronometre Observatoriumsuhr
4743020 Cal 135 Zenith Chronometre
4743033 Cal 135 Zenith Port Royal Chronometre
4743050 Cal 135 Zenith Chronometre 9415400
4743106 Cal 135 vom 30.10.1959 Zenith Port Royal Chronometre 765526 18K Rössler
4743115 Cal 135 Zenith Port Royal Chronometre 9483935
4743131 Cal 135
4743195 Cal 135 vom 13.05.1959 Zenith Chronometre 9385925 Rössler
4743437 Cal 135 vom 18.11.1959 Zenith Chronometre
4743679 Cal 135 Zenith Port Royal Chronometre 9493959
4743724 Cal 135 Zenith Port Royal Chronometre
4743730 Cal 135 Zenith Port Royal Chronometre 816530 18K
4743803 Cal 135 Zenith Port Royal Chronometre
4743921 Cal 135 Zenith Chronometre
4872096 Cal 135 vom 07.09.1960 Zenith Favre-Leuba Chronometre 9564716 Rössler
4872109 Cal 135
4872349 Cal 135 Zenith Port Royal Chronometre
4872480 Cal 135 Zenith Chronometre 700160 18K
4872674 Cal 135 Zenith Chronometre
4872857 Cal 135 Zenith Chronometre 9663247
4872882 Cal 135
4872943 Cal 135 Zenith Port Royal Chronometre 9651873
5362134 Cal 135 Zenith 2000
5362178 Cal 135 Zenith 2000
5362197 Cal 135 Zenith 2000 9669117
5362590 Cal 135 Zenith 2000
5362625 Cal 135 Zenith Chronometre Port Royal 9697655
5362627 Cal 135 Zenith 2000
5362666 Cal 135 Zenith Chronometre Port Royal 9697666
5362680 Cal 135 Zenith Chronometre Port Royal 9697590
5414172 Cal 135 Zenith 2000
5414383 Cal 135 Zenith 2000
5455034 Cal 135 Zenith HAU
632xxxx Cal 135 Zenith 2000 Chronometre
6326344? Cal 135 Zenith 2000 Chronometre
6326351 Cal 135 Zenith Favre Leuba
6326479 Cal 135 Zenith 2000 799A648
6326611 Cal 135 Zenith 2000
6326627 Cal 135 von 1957 Zenith 2000 Chronometre Rössler

 

[Hartmut Richter]

Nope - 'fraid not! The German version is correct: it states 11000 made (rather than 13500). I think that was a bit of a Freudian slip on behalf of the translator - take the calibre number and add two zeros! A pity - it would have amounted to an increase in production of ca. 20%.....:-(

Hartmut Richter

 

[sempervivens]

Nope - 'fraid not! The German version is correct: it states 11000 made (rather than 13500). I think that was a bit of a Freudian slip on behalf of the translator - take the calibre number and add two zeros! A pity - it would have amounted to an increase in production of ca. 20%.....:-(

Hartmut Richter

Thank You Hartmut! Translators are dreamers, aren't they, dreaming their day away.

However it's still a 13 lignes movement that was produced for 13 years

Hi sempervivens.
I have added your numbers to my list:

Thank You Hans I've added your numbers to my list as well. Thanks for all the extra work ;-)

 

[sempervivens]

Thanks to Hans, there were also several observatory chronometers included in the list.

These were not for sale, made only for testing purposes.

The observatory cal. 135-0 movements always appear goldplated without côtes de genève.

They have a normal regulator instead of the fancy old (1903 Zenith patent) regulator.

No incabloc, hence it is named cal. 135-0 instead of cal. 135-6.

And they always have the "decoupling device", revealed by the "pin with round head" near the crown wheel, as in the first series cal. 135.

One has # 4641318 and can be seen on this page https://sites.google.com/site/zenithistoric/zenith-et-le-calibre-135 :

The serial number dates it to around 1956.

Another observatory chronometer which can be seen on the same page has a later number # 4732508 (around 1958).

That is close to three more observatory watches reported by Hans:

4732528 Cal 135-O Zenith Chronometre Spezial Observatoriumsuhr
4732661 Cal 135-O Zenith Chronometre Observatoriumsuhr
4732666 Cal 135-O Ebauche 1950 Zenith Chronometre Observatoriumsuhr

This group of serial numbers dates to 1958, or just prior to the launch of the Zenith Port Royal.

The # 4732528 can be seen on this page Hauke Heffels - News (entry dated 26 May 2008).

Maybe Hartmut can translate the information by Heffels about the "isolateur" or decoupling device?

This, we are informed by Heffels, has an observatory bulletin dated to 1960:

J.P. Sunier regulated it and it received an N = 13.

 

[Hartmut Richter]

It is close to midnight, I have just eaten my dinner after spending ca. 2 hours putting a new computer table together (after putting the children to bed, after repairing one of the children's toys) - so I'm afraid that the translation will have to wait until tomorrow! You seem to have got the gist pretty well yourself.....

Hartmut Richter