Mathis, thank you very much for your reply.

I really don´t think that the watch is a made up from different parts for two reasons:

first:

I got it from its former owner which is relative of mine and he had it since new and stopped wearing it when the crown broke off.
This happened three times before and he always had it fixed.
But after the fourth time with a lost crown, sometime in the 90ies, he was finished with handwinding and breaking off crowns, so after he had it fixed again he put the watch away and bought a Tudor Big Block which „winds itself“ ;o)

I even know the store where the 13-1 was bought back then.
So I think I can be sure that the watch has not been doctored with, besides that it had seen the watchmaker four times in its life (there are also 4 watchmaker marks visible at the caseback).
And I have been told, that all the times when it was with a watchmaker, the only thing that was repaired, was the crown (and stem i suppose).

For me, the watch looks, except for its crown, all original.
Okay, there is no original strap, but to use the words of the former owner: „the strap was a piece of plastic junk - it broke after 2 months“.

second:

There are too many 13-1 out there with this exact caseback style.

Here another one which has been sold on a swedish forum:

https://klocksnack.se/threads/lönebump-heuer-pvd-kronograf.96910/

And again, this one here really is an excellent example of this variation of a 13-1, it even has the original bracelet.
Like I said before: I have a picture of the caseback, maybe the owner can chime in here can confirm this:
http://chronocentric.com/forums/chronotrader/index.cgi?md=read;id=94691

Regarding your note that it looks like there is a gap between the dial and the tachymetre:
there is no gap between the dial and the tachymetre scale.
Maybe you are referring to the thickness of the tachymetre scale which looks a bit dark in the pictures.

Please have a look at the new detailed picture I just took, I hope this shows the tachymetre scale a bit better.
The lighting situation is not the best at the moment.

And please have a look at the other pictures (which i collected from the net) I attached.

Heuer did not produce these casebacks, they were bought from the case maker with the watchcase.
These were standard watchcases, delivered to a lot of other brands, sometimes equipped even with the same dial design as the Heuer 13-1 only with another brandname printed on the dial.

There are several 7765 chronographs out there with the „13 1“ from the first variation on the caseback which are not Heuer branded.
Instead they were branded: with LeJour, Arctos, Lanco, Zentra Le Mans to name a few.

For me it looks like the first variations (the ones shown at OTD) are the older ones and the second caseback variation are the latest execution.

And this I think is the interesting point:
I can not find any other 7765 chronograph with the second variation caseback except the ones from Heuer.

Greetings,

Hermann