This round and the gun to go with it were designed by the French branch of Hispano-Suiza just before WW2, under the designation HS 411. The cartridge case seems to have been based on the Hotchkiss 25x163. Drawings were sent to Hispano's British subsidiary BMARCo probably around the fall of France, and one gun was made plus several batches of ammo (all 1942 dated). The gun was offered to the MoD who couldn't see a use for it.
The Swiss branch of Hispano took over development from the French, and in the 1950s it emerged as the HS 830 (later HS 831). Early cases were brass, later steel took over. Since 1970 when Oerlikon acquired Hispano it has been known as the Oerlikon KCB. It did eventually make it into British service: after the Falklands War single and twin 30mm mountings were bought for the RN, and some are still in service.
Earlier, the British had adopted the round for the Rarden AFV gun, but reverted to a brass case. It's always worth checking the headstamp of Rarden cases, just in case they are not Rardens!
