Doverham--You've got a gun with a double St. Etienne proof (mark seen twice on each barrel in front of the flats) and with factory 2 3/4" (70mm) chambers. The chamber length would lead me to speculate that it's perhaps more likely to be a post-WWII gun than one from between the wars years. Acier diamant = diamond steel. Pretty much like the different names for the steel Parker used in its doubles. Should indicate higher quality steel.

What you found on Helice, Helicobloc etc is a good explanation of the situation. There are a lot of French guns out there not made by V-C which use the same system, or perhaps minor variations of it.