This rifle has some oddities and, unless the barrel signature can be cleaned up and deciphered, may remain an unknown gun. There were many small, backwoods gunmakers in southeastern Kentucky and northern Tennessee [where details suggest this rifle was made] who were trained as gunsmiths and blacksmiths, but often did more farming, milling, carpentry, tinkering, etc. than gunmaking. The infrequency of viewing their work along with a lack of unique details make it almost impossible to identify unsigned rifles. Some of them never signed their rifles, perhaps due to being considered more a tool than family heirloom to families in the hill country. My thoughts on the rifle, based on its several more distinctive details, are as follow:
Overall, I think this rifle favors a Tennessee origin more than a Kentucky origin. The side facings are rather large with significant wood in front and behind the lock plate. Most Kentucky makers kept their side facings rather tight around the lock plate, while in Tennessee you see longer side facings. The cheekpiece is also longer than normal for a Kentucky gun, and the lock bolt washer is a form I have not seen on any KY gun before...an exaggerated comma or tear drop shaped washer. Most telling may be the guard's grip/finger rail that butts squarely into the back of the bow, rather than a lapped weld. This detail is not, to my knowledge, used on any southeastern KY rifles, but is used on a number of TN rifles. I have seen guards in this general shape/outline made in KY, even with the TN-style curl in the rear spur, but I've never seen a KY iron guard where the grip rail butts directly into the back of the bow.
The rear trigger is somewhat odd for a KY gun, swept back somewhat more like a shotgun trigger. That detail may also be odd for TN, but rare in KY. You do see in southeastern KY guns that are iron mounted, the rivet attaching the toe plate to the toe of the butt plate as on this rifle, but it is probably more common in TN. The short tang is also somewhat odd for a rifle from this hill country area...most use a longer tang, often with two screws, and at times a third small one in the tail of longer tangs.
The percussion side lug with its extended square shank is often seen in KY, but at times elsewhere, and probably indicates more a "southern rifle" than a specific KY or TN rifle. One other point is the barrel, which apparently was originally full-stocked due to the visible old barrel loop scar under the iron rib. Perhaps the gun was originally full-stocked, but hill country guns, due to the poverty in the general area, used recycled parts more frequently than guns made in more prosperous areas.
If it were my rifle, I would work on the eroded barrel signature, probably a little more aggressively than most people would advise, since identifying a few of the letters to possibly determine the gunsmith's name, or at least part of it, may be the only way left to tell where this rifle really came from. Shelby Gallien