During the 1930s, SIG Neuhausen made a series of really beautiful submachine guns. They were the MK series, offered in either 500mm / 19.7 inch barrels for military use or 300mm / 11.8 inch for police use. The first pattern was the MK-O, which had a rate-reducing system built into the action (which looks initially much like a delayed blowback system but isn't). These proved too expensive, and a simplified plain high rate of fire model was introduced, the MK-S.

This example is a very rare factory semiautomatic model purchased and imported by Sam Cummings. The vast majority of the MK guns were fully automatic, but semiauto was a factory option, as police use was one of the intended markets.