That is classic upper Susquehanna and exceptionally well made to boot. I have not particularly focused on the makers in that area other than the Baums, however, so I'm not sure exactly which one it is. I'm sure someone will be along to put a name to those initials. Beautiful rifle and at first glance it looks like it *may* be original flint, which would be one heck of a big bonus. I'd like to see that lock closer, in detail, to determine if it was reconverted. One thing I do wonder about it the somewhat abrupt muzzle work - like the front rammer pipe, the end of the foreshock and muzzle cap are all crammed together. I wonder if it might have been shortened by a few inches at some point. Regardless, that is a really wonderful example of an upper Susquehanna piece.