That sure is an interesting old rifle!
I am inclined to agree with
@Grenadier1758 regarding the percussion lock, and replacement of the mainspring. It looks to me as if the stabilizing pin on the side of the new mainspring was in a slightly different location from the original, so the gunsmith drilled a new hole for it and left the old hole open.
That is a very unusual lock. The hammer is elegant, but of a very unusual form. The shape of the lock plate is reminiscent of some early flint locks, but the plate is dead flat with no ornamentation or markings of any kind. By the time original percussion rifles were being built, "factory made" and "Warranted" locks were commonplace. I'm wondering if that lock might have been put together from assorted parts, on a "shop made" plate. Another thing is that it has two side nails, which is usual for flint locks, but much less common for percussion rifles.
Very perplexing.
I would agree that the barrel has been shortened a few inches. The forward ramrod pipe overlaps the nosecap slightly, while we would expect it to be several inches back. I would conclude that the barrel and the wood forend were shortened, and the nosecap refitted to the shortened stock.
The question might be, "Which end of the barrel was shortened?" A lot of old rifles had more or less erosion at the breech, inside and out, so they were shortened from the back end and a new breechplug was fitted. The barrel was then moved back in the stock. If the barrel from this rifle could be taken off, turned over, and examined, we might find some old dovetails hammered flat, to show where the tenons had been moved. I just have a feeling that whoever shortened the barrel would have been less likely to ornament the muzzle than the original builder.
One last thought is that the cheek piece appears rather crudely carved. It just doesn't match that elegant patchbox.
Regarding the abbreviation of "Kentucky," I have few facts to offer.
@Davey Boy has advised us that the USPS adopted official state name abbreviation in 1963. I didn't know when that occurred, but now we do! However, I do know writers back in the day used abbreviations, and I have noted the use of a small, underlined letter in superscript, as on the "y" in that barrel's marking, in some very early 19th century writing. I would not know how to use that for dating the rifle, though.
There is much to ponder with this rifle. Thanks,
@Celticstoneman , for presenting it!
Notchy Bob