Rare Compax model? Chrono made for Cartier

Yes, that's why I asked. Often extra serial #s on the bottom of the lugs.

No, there is nothing to find.

Could you find out something about this Cartier UG?

Myself I couldn't manage to find any new information 🙁

Yes, I finally spoke to my watchmaker yesterday. He was trained by UG in Switzerland and brought to the U.S. in the 1950s to service UGs in New York. He said he clearly remembers Cartier using Cal. 42 UGs in that era (time only watches for women). His wife worked for UG also, and she was responsible for sending the dials for those Cal 42s out (presumably to Cartier) to have them signed. He said he doesn't remember any chronographs, but that wasn't his department. He said it was very probable that UG did chronos for Cartier.

Dear Members,

I recently took a close look to case bottom lately. I think, that it once was engraved
and that this engraving was filled up before grinding and polishing the bottom, but
probably the complete case.
This morning I asked a friendly goldsmith master if she knows a way to make such
filled engravings vivible again. No answer so far, but maybe this could be the hint
why there is no Cartier number to find.