i tried with a wrench but didn't use a holder and fail to open it . i think it is a 8541 as it doesn't hack.What did you try to open it with? Rubber ball (works on not particularly tightly closed backs, and doesn't leave scratches), or a wrench?
Screw-in backs can sometimes be tricky to open- are you using a watch holder in the process?
Regarding whether or not it's the B version, does it hack? The cal. 8541 was also available with the hack feature, 8541B always had the hack feature.
My bet would be the 8541, the B version was introduced in 1970, and this piece screams 1960s. But then again, IWC did a lot of rather conservative designs even in the early 1970s...
I want to second this suggestion. It seems to me that many of the problems people have opening case-backs are actually related to the difficulty in holding the watch. It doesn't need to be fancy. I have this one, and it works just fine for most applications.Get a watch holder, then- it's very useful. Especially that it reduces the risk of scratching the back. Just holding the watch in hand is no good, as a hand hardly provides a pivot firm enough for a wrench to do its job properly, especially if the back is screwed in tightly.