Mathis, thank you very much for your reply.

Interesting discussion, thanks for that.

Please have a look at the pics of some of the others I found with the exact same caseback style attached at the end of this post.

I really don´t think that all these watches are put together.

Because - and this observation is a really important one when we discuss this:

I can not find any other 7765 OR 7750 chronograph with the second caseback variation (13/1) except the ones from Heuer.

This is a very important point - which should not be overseen when discussing the 13-1.

I can find a ton of the other variation (13 1), which has been used by a lot of other brands.
I showed some examples of that in my other post.

But, again, there are no other examples for the 13/1 except the one from Heuer.

Which brings me back to this:

It looks like the Ref. 13-1 had at least 2 caseback executions (first: 13 1 and second: 13/1).

I am looking forward to read some more opinions on this topic.

Thank you very much for your help.

Hermann