I suspect the initials on the lock where those of a previous owner of the gun, and not those of the maker of the gun. From the decorations on the patchbox and cheekpiece inlay it's clear that the gunsmith knew how to use an engraving chisel. The stock profile (forward nose of the comb shallow & tight to the wrist area) looks Huntingdon County, PA to my eyes. It's likely that Shreckmeister already has a bead on the maker of this gun. Looking forward to him filling us all in.
Loudy