I think this gun might be included in the catagory "forever a mystery". Lol.
I think some individual built this gun from some surplus parts sometime in the 20th Century. From the photos the Inletting of the metal to wood parts looks very good. Not semi-mass production. And the contour of the forearm of the stock to the muzzle. This was built by someone that knew what they were doing. The ramrod looks like it was turned from bar stock. Not a forgeing. The butt plate, trigger guard, trigger, and back action lock look typical of late English/European/American percussion fowlers. The barrel, with it's late style stampings is the big mystery. If the breech were inspected, it would not surprise me if the barrel was in firing condition.
While not a big fan of back-action locks, I like this gun. Whoever assembled it did a good job. Rick.