The following paragraph is taken from Tony Williams website:
This was developed in Switzerland in the late 1930s for the FMK aircraft gun (which saw limited use) and the Flab Kan 38 AA gun, which remained in use for some time after the war. This cartridge is very similar to the postwar 20x139 cartridge for the Hispano-Suiza Hs 820, but the rim is thinner and the cases of brass."
I have a cartridge case which appears to be for this cartridge but it is a steel case with brown lacquer.
The headstamp is: 284 46 D A with a white paint stencilling: 273-46A X. The screw in primer is marked: 312-46A X DC (the D and C are overlapping each other and there is a dot inside the C. Any ideas please?