The pics are of my 750.501 that I purchased new in 1980 complete with boxes and papers. It has the case that most Lejours have rather than the curved edge bezel of most Heuers. I sought Chuck Maddox's input and here is a quote from his reply.

"I suspect that by the time these models were in production, Heuer was in dire enough financial state that Heuer was happy to be making watches for others. Remember that Lenamia owned Heuer for a while before TAG bought them out. Usually your suppliers don't own your firm unless you've fallen behind in Accounts payable."

The link below is to the entire discussion and should answer at least some of your questions.