I would think this rifle started life as a late period, full length, full stock, flintlock, made in the Upper Susquehanna region of Pa.
My reasoning; If the trigger pull measures around 12 1/2 to 13 or so inches long that's about the norm for a full stock rifle.
If you look just behind the percussion hammer you'll see a notch cut in the wood for the original flint cock, when pulled back to full cock position. Also notice the filled in holes in the forward part of the lock plate.
As for the barrel length, if you measure the distance from the rear sight to the normal rear sight position near the ramrod entry pipe, that's the distance the barrel was cut off from the rear. The normal distance for a rear sight is about 10 or 12 inches from the breech.
The normal length for a barrel from this time period is around 40 inches, give or take some. If the length you find cut off from the rear, when added to the current barrel length doesn't get to about 40 inches, then the missing amount is what was cut off from the muzzle end. I doubt the punch marks on the muzzle are original, as I don't remember seeing them in such a haphazard manner.
Just my opinion of course, and your mileage may vary.
PS, I have a fancy one just like it leaning in the corner of the room, signed S Miller on the barrel, in case you want to start a collection of cut down rifles!