The round is a 30x210 for the MK 303 Br canon for the type XXI submarine. One canon is in the military museum in Prague. Its barrel No. 100 is stamped 30mm Flak M 44 U (U for U-boot?) . The number built is 32 in 1944 and 190 in 1945. There was a zwilling submarine variant, a single mount for surface ships, a single towed ground variant (for these pictures exist) and supposedly there was a twin flak on Pz IV chassis. (source Strelecka revue 4/2007)
The canon developed after WW2 into Czech 30mm PLDvK.
The projectile has no markings and from what I could measure it corresponds with 3cm-M-Gesch.Lspur Üb.o.Zerl. Rotating band is steel. It was filled with orange clay-like substance (needed to measure the internal cavity),probably inert (still here ).
The fuze is a AZ-1504A not the correct one. Should be zündererzatzstück
The steel case was manufactured in Povazska Bystrica (dou on 9) in 1944 (on 3) the waffenamt is WaAA80.(on 4). 3-303St (on 6) and bzs. and 153 (on 12) probably give a model and lot.
The round was used for PLDvK loading training mixed between newer czech rounds.
I will be thankful for any information on the ammo for the BK 303.