20 mm Oerlikon cartridge 1943 marked "1943 Oe Z RG" INERT This type was used for anti-aircraft in WW II in the Oerlikon and Polsten machine guns. The Oerlikon 20 mm cannon is a series of autocannons, based on an original German 20 mm Becker design that appeared very early in World War I. It was widely produced by Oerlikon Contraves and others, with various models employed by both Allied and Axis forces during World War II, and many versions still in use today. The Polsten was a low cost Polish development of the 20 mm Oerlikon gun. The Polsten was designed to be simpler and much cheaper to build than the Oerlikon without reducing effectiveness.