If you moved the key out you should be able to remove the barrels from the action. Then look to see if there are any proof marks on the underside. That would give you confirmation of British or Belgian origin. Made in the US you would find no proof marks. If unclear how to move the key go to YouTube and watch it done there on black powder muzzle loaders.
Another thing to do is to make certain there are no loads in those barrels. Seems like half the old family muzzle loaders seem to have on or both barrels still loaded. Get a long wooded dowel and mark it from the muzzle end to the rear of the barrel. If you drop it down the bore and it is a couple inches too long consider that gun to still be loaded. Black powder will last a long time.
