Here are some observations:
-- one of the chamber measurements looks to be "6.5". That would make it pre-June 1912. The other doesn't have the decimal but the "6" and "5" look strangely separated.
-- In addition to "acier rationnel" there is "Acier Comprime". That is classic translation of Whitworth's "compressed steel." It was used on a lot of Saint-Etienne barrels. Here is a "dolphin" trademark with it.
Ronchard Cizeron used it a lot.
-- It looks like the initials are "CM" to me. Can't identify who this might be and I have no record in my Saint-Etienne stamp database." My book on Saint-Etienne gunmakers arrived back home while out in Africa. Will have to wait till I get back to review it.
-- There are a lot of French hammer guns out there.