No he did not. I own several watch before. I can sure the authentication since because except this issue, the rest is perfect.
The watch also come with full box, 3 cards, and he sent it to service with paper as below (it is in Korean so I translated it). I also own several watches and basing on the finishing, I pretty sure it is authentic
I just only wonder why Omega come out with this mistake
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If this does help, the address and the AD name does point out to one of the OMEGA AD in BUSAN, south Korea, so the watch does look AD serviced. authentication is probably a least concern right now. Sure, the reciept could have been fabricated, as well as the watch swapped wit hcounterfeit with same serial number, but I personally doubt anyone will go that far, esp. with likes of an Omega. (if it was a richard happy meal, maybe...)
I would love to see the original text for "coagulation contamination" in Korean, as the translation admittably is a bit weird.
Defect wise, OP's pics were a bit vague, but the "-" on SWI-S made looks to be located on the top, which makes me speculate that either during service, or via simple wear and tear, the "s" just fell off. More likely the former, esp. if this watch's dial was "cleaned off", per servicing process. Likewise, I feel like if this watch was erronously printed, the "-" may look cleaner. That said, I am sorry to say your PO is likely not the factory "error dial".
Now when it comes to "disclosure", ideally the seller will take refunds, or at least be reasonable with you, if you have some complaints regarding this issue. However, imho, if seller did post all the detailed pics of the watch, which did indeed display "swi-s made", with comments like "please see pic of the watch for details", I still think seller did his best trying to disclose everything, and won't be surprised if this "miss" is likely an unintended peccadillo from his end.
Should you have indeed covered your ass and saved the sales post, as well as the pics of the watches from the sales post, I say look for the "-" in those pics. if posted sales clearly show an "s", then we have a problem. Otherwise, this "-" is likely one of those classic "**** happens", with used goods purchase. If the identical "-" is detected from vendor pics, then I think the seller, though not perect, has done his part trying to disclose much as possible, and some responsibilty is in OP's hand for not recogniziing it them.