You may have to remove the barrels from the stock and look on the underside of the barrel close to what would be considered the breech end, if it had a breech. There should be some Proof Marks there and if so and researched, it would at least tell you who the made the barrels which might just be the Maker of the shotgun. In any case, someone would likely pay $100 to $200 even if the Maker cannot be determined and possibly more if it is shootable.