This is a 12 Gauge Grade 1 Robust, i.e. the lowest grade. It is I would say dating circa 1920s.
It was of course made by "Manufacture d'Armes et Cycles de St-Etienne", much later known as Manufrance.

Here are the markings:

Charge d'epreuve fusil fini
proof load, finished gun: T powder 3.46grams 36 grams of shot (~1 1/4 oz)

Charge normale
Service load: T powder 2.2grams, 32 grams of shot (~1 1/8 oz)

Acier : Steel (e.g. not damascus)

Choke perfectionne: Improved choke (in the sense of better, not the choke opening). The right is 1/2 choke, the left is full choke.

Fabrication mecanique: tooled manufacturing, not done by hand

Canon frette: sleeved tubes

Hope this helps...
Crazy Bull