United Defense UD M42 submachine gun has been designed during 1941-42 timeframe by Carl Swebilius, American designer who worked for High Standard Manufacturing Company (USA). First UD M42 submachine guns were made by High Standard and Marlin on Dutch order, and most were shipped to Dutch East Indies just prior to Japanese occupation. The rest of about 15 000 M42 submachine guns made was purchased by US government and mostly used by OSS personnel (Office of Strategic Services, the predecessor of CIA). The UD M42 was somewhat complicated, but well-made weapon.
UD M42 is blowback operated weapon and fired from open bolt. It also has separate firing pin that was operated by hammer. Charging handle was located at the right side of the receiver and stayed still when gun was fired. UD M42 used box magazines with 20-round capacity; unusual (for the time) feature was that two magazines could be clipped together in front-to-back, upside-down manner to provide user with spare magazine handy.