I can't offer you any detailed info on your gun, but I did own a Manufrance-St. Etienne, made double barrel a number of years ago.
As I remember, Manufrance is the marketing name, St. Etienne is a state armory/ manufacturer.
I think some were imported in the 60's, 70's and maybe the 80's, but not in great numbers. There were some very fancy grades offered. Mine was a plain jane 16ga. sxs.
I don't think you'll find them commanding a very high resale value, unless you can find some die hard French collectors(there are some out there, but most are ashamed to admit it
). Parts would be hard to come by too.
I should add, I had it about 5-6 years ago and only paid $150. for it.