This is extremely intriguing!
I don’t think I’ve ever seen such a Heuer model - in catalogues, picture or any sales platforms.
I also am firmly convinced that Heuer did not start using Lemania movements until they were bought by them. I am baffled by the various places your watch is signed: the shield on the dial is very well made, but the one on the caseback inside not so much - though they sometimes are also on watches that are known to be certainly genuine.
Furthermore most brands that did use the 1341 did not engrave the oscillating mass as far as I can remember. And the use of this movement- which was costlier than its successor 5100 - pre-dates the “parts bin” days considerably.
All things considered, I am torn between non genuine or something of a prototype that never made it to a production run. Either way, your watch is very nice, and the movement is a high-quality one. So aside the very natural curiosity I’d suggest enjoying it as is, regardless of whether you’ll be able to ascertain its veracity for sure.
Cheers,
Fabrizio