The "U" on top of the receiver is not a Definitive Proof mark as stated in the note shown in the photos. The rifle was made with a leaf rear sight on the barrel which was removed and the dovetail filled. I suspect the matted top of the receiver is to cover up a filler for another dovetail cut for a claw mount front base. In this case the "U" may be the marke of the maker (UTRECHT?), retailer, engraver, or something else altogether. A bring back rifle with a dovetailed base for a missing scope is pretty common and filling the dovetail is also a pretty common way to address the problem. I have filled a couple, myself. The addition of a peep sight would also be a common period, American, solution. This is all only supposition to explain the unusual "U" on the receiver.
Mike
