Originally Posted by desconhecido

Well, it's not a terrible bicycle and looks worthy of being repaired and ridden. I'd guess that being 82 or 83 that it might not have the French standard dimensions on things.

Problem with replacing that rear derailleur is that the bike appears to have decent dropouts with, probably, a Simplex derailleur hanger which, if it's like the one on my Peugeot PR10, is unthreaded and without a pivot stop. The hole can be tapped and some people suggest filing the dropout to fashion a pivot stop. You might consider giving the old Simplex derailleur a try. They didn't shift bad, in my experience.

edit: Simplex derailleurs are held onto the hanger with a hex head screw on the back. You probably already know that, but thought it was worth mentioning.