Your gun is a French 12 gauge that was double proofed with French proof powder T. It was definitely built post 1900, as that is the year that powder started being used at the proof house. It originally had 2 1/2” chambers, they may have been lengthened by now. I doubt it was used in WWI except perhaps to acquire game for the table. That action was used on many French and Belgian guns, and was produced in France for the gun trade.
Gene or Fab probably know who built the gun. It should be inspected by a good double gun smith, who will measure the chambers, examine the barrels for corrosion and pitting, and advise on proper ammunition.
Good luck.
Best,
Ted
