As I mentioned above. I wonder if this was one of the first shotguns made after Nov 11, 1918. I think the whole Saint-Etienne industry concentrated on the war effort and not on making sporting guns. This is mentioned in an article about the Berthon Bros patents. Is that other number the serial number of your gun? Is there a name on your gun? If so, you've come pretty close to dating it.

Baluch are not Brahui, Brahui are Baluch