( 30mm case for comparison )

 The 2-pounder Pom-Pom Gun, officially designated as the QF 2-pounder Mark VIII, was a British naval anti-aircraft gun widely used during World War II. The associated cartridge shell case for this gun, specifically the No. 1 MkII, had distinct characteristics. Here are some details about the British WW2 1937 Naval 2-Pounder Pom-Pom Gun No. 1 MkII cartridge shell case: 

The 2-pounder Pom-Pom gun was widely employed by the Royal Navy and other naval forces during World War II. It was a crucial component of shipboard anti-aircraft defenses and played a significant role in protecting naval vessels from air attacks