This vessel carries a modern 20mm (?) gun. Picture is taken between mid 1950s (likely).
Its a former KFK into service as a guard ship in the Federal Germany Navy.
Due to the bridge-wing style (postwar rounded wings) + radars, picture likely taken during 1960s-early 1970s (after probable late 1950's-early 1960s refit)
I'm fairly sure that's an early 20mm S-20 Marinelafette.
I concur, maybe a 20 mm/65 Rh202
prototype ?