One observation, which will not help with identification, is that the nipple appears to be an old one but it looks like one for a sporting arm. The cone is smaller, to take a "civilian" sized percussion cap, or at least it looks that way to me. I would think a military arm would take a musket cap.
Nowadays, percussion caps for sporting arms are pretty much limited to #10 and #11, but years ago, they came in a much larger variety of sizes. This could really complicate supply for military applications. Musket caps and musket nipples, as far as I know, were much more uniform in size, to simplify supply. Nipples on surplus guns did sometimes have the cones turned down at some point to allow the use of sporting caps, but to me, the base of this nipple also looks small. Musket nipples typically have a 5/16" or 8mm threaded shank and correspondingly sized base. Sporting nipples would more likely be around 1/4" or ~6mm.
If I am correct in this assessment (and I could be wrong... It's been known to happen... Just ask my wife!), it would suggest to me that this gun may have been surplus, or the nipple may have been replaced at a later date.
Post #3 says something about rifling and "rear elevation sights," but I don't see any sights on this gun and there are no photos of the bore, so I don't know if it has been rifled or not. I think a smoothbore configuration and a simple rear sight, or no rear sight, would be more desirable in a gun destined for the surplus market and civilian use, to allow the use of bird shot, buckshot, or ball ammunition.
Respectfully suggested,
Notchy Bob