I think we may have jumped too quickly to the conclusion this is an original back-action lock gun and therefore has always been percussion. If you look closely at the wood in front of the lock plate, you can see a large, rectangular splice has been installed, probably to fill an earlier front action lock mortise. That also helps explain why there is a standard shaped side facing on back side, the poor woodwork and fit of current lock, and what appears to be another patched-in splice just above the current lock plate, with several small iron nails holding all the spliced "stuff" together... and what seems to be a cock cutout above the lock plate.
I believe this gun originally had a front action lock, and while a single bolt attachment [unless a front plugged bolt hole has been cut off the left side of the picture in 3rd pic down], it could have been an old recycled flint lock. Down in Kentucky, single bolt flints started showing up about 1820, and probably earlier back east. Shelby Gallien