Definitely German proof marks,

Crowned 'U' represents Untersuchung and is definitive proof on guns in final state used 1891 t0 1939. Was replaced in 1912 by a Crown N or 'NITRO', but was still valid until 1939 as final black powder proof.

The Crowned 'S' and Crowned 'W' German proof used between 1891 to 1945 and are proof on smoothbore barrels and proof on choked barrels, respectively.