Nesredep, thanks for posting. It's an ink stamp, dimmension is 40x55 mm. It's the City Weapon of Swinemunde (until 45), today Świnoujście in Poland. It was found and made in Trondheim Norway and came together with several KM items. Several U-boats used a similar wappen and one of them was allocated to the 13th U-flotilla, but who knows, I'am still investigation and this is only a theory, all suggestions appreciated , cheers Frode