The stamps around the breech have me wondering if someone used an original barrel with a replacement breech plug in a trapdoor stock to build themselves a rifle. Awful lot of stamps there, more typical of older gun barrels I have handled than modern production ones. The plug appears to be cast, not an original machined military part, so I'm guessing a modern era replacement. Basically thinking someone had some parts and wanted to build a Civil War type musket for their own use. I remember the Numrich kits Hawkeye2 mentioned from years ago, no reason someone couldn't do their own version using what they had available.