The 14 stamped on the barrels of your gun should mean that it is bored as a fourteen gauge. If it chambers a 12 gauge shell something is drastically wrong!!!The gun looks to be of Belgian manufacture, despite the partial British proof marks.The use of fake British proof marks and makers names, was a significant problem for the British trade at the time this gun was made.
Looking at the condition of the gun I would consider it to be a wall hanger. Hope you are not too dissapointed.

Roy Hebbes