I think it is a practice shell for the Dutch 12 cm naval gun L/50 (Bofors), which was a captured gun in use with the German Navy. Dutch shells with copper driving bands and brass cases were used by the German Navy, but later in the war the Germans produced their own shells with iron driving bands and steel cases.
I don’t know the official German designation of the gun, but the German steel cartridge cases were designated 12 cm Patrh. 41 St (H) or in short in the head stamp: 12 – 41 St (H). Case dimensions are 120 x 720 mm.
The shell you are showing has some long arrows stamped in, which means it was an inert loaded shell for practice use. German naval practice shells were painted red hence the red paint in the stampings.
Both driving bands lack profile, so I think it was a recovered fired shell from which the rifling in the driving bands was turned off on a lathe.
Below a picture of 3 cartridges for the 12 cm gun. From left to right: a HE shell fitted with a Zt.Z.S/30 and iron driving bands on a steel case, then a star shell fitted with a Zt.Z.S/60 nA and iron driving bands on a brass case and at last a practice shell fitted with a zinc transport plug and copper driving bands on a steel case.
Also a picture of the headstamp of a steel case. Note that the electric C/22 primer is not correct, this must be a C/12 primer.
The pictures were taken at the Warmuseum Overloon in the Netherlands.