gloftness,
The crown U is the mark for the "view" proof, which is a detailed inspection,usually after the definitive proof.The 16 in a circle means it is chambered for 16 ga 65mm( 2 1/2-2 9/16") shells rather than current standard 2 3/4".If the "nitro" you mentioned is written in script, the gun was likely proofed in Zella-Mehlis,if block letters-then Suhl.It would have likely been made in or around the town in which the proof house was located.
Mike