Its an original, probably from either Ohio or Western Pa. The back action locks were commonly sold through hardware stores in the post civil war period. Lots of these kinds of guns were made by local buidlers. Have you looked the underside of the barrel, the inside of the lock plate, and the inside of the trigger guard and buttplates for any indication of the maker, or date of manufacture? Sometimes you can get lucky finding an initial or a name, or part of one that will lead you to the name and approx. date it was built.
Check the condition of the underside of that barrel, which is covered by the stock. In the early first half of the 19th century, when the transition was being made from flint to percussion, barrels made for flintlocks were sold cheaply and used many years later for thes kinds of low cost "farm guns " with percussion locks. The cheapest locks then available were these back action locks, which were often made in Belgium, and could be bought in pairs to make a DB shotgun, if someone thought he could make the barrels and regulate them correctly. Most of the locks were used as you see here.
The gun looks like it is in good condition. You really need to take it to a gunsmith to tell you what the actual caliber is. Then, he can also check out the condition of the bore, and indicate whether it is still safe to fire. You can also get replacement nipples for those guns, so there is really no reason NOT to shoot it.
My brother has an OHIO gun like that, made up in the 1870s, or 1880s, but the barrel dates back as early as the 1790, or early 1800s, and was originally on a flintlock rifle with full stock. YOu can see the dove tails in the barrel where keys were used to hold the stock to the barrel. Now it is a half stock, with a false rib, all of which was done to the gun about 140 years ago. He restored the wood stock, and has since had to do extensive restoration work on the patent breech, twice, to keep it shooting. The barrel had to be bored out to a larger caliber to remove some very deep pits, but Bobby Hoyt did an excellent job, and the new caliber( .42) shoots RB to the same POI as the original sights on the gun are set to do. The barrel is of iron, the only iron barrel he has in his collection, but it still shoots very well.
