You are right, it certainly looks like a Heuer sourced case. I would be pretty certain it was a ref 406. One tell tale sign for me is the unusual wide spacing of the serial number digits, which I have seen on a few of the three register A&Fs. Also the back case inscription looks correct.

I'd be interested in the view of Jarl RE, as he has studied Heuers of this period. Jarl, if you are reading, what do you think?

The dial is ugly though (I hope you don't mind me saying that). It would be great (if difficult) to find a replacement. Maybe advertise on Chronotrader; what may help you is that even a worn dial would look good, so maybe someone has one that was replaced lying around...

Nice find, I have seen that watch advertised on eBay but because of the refinished dial and the deceptive camera angle I dismissed it as a fake without looking. Good work!