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 mark of the maker (UTRECHT?), retailer, engraver, or something else altogether. 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
There is absolutely no evidence of a front receiver cut for a claw mount. And no evidence of any alteration of a corresponding rear claw mount base. The rear sight filler block is also period German and not a later U.S. addition. Here are two examples of German rifles in my collection that also had original Lyman rear sights - added by the factory at the time of manufacture.
https://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=645776&page=1https://www.doublegunshop.com/forum...r=643075&Searchpage=1&Main=28089
