The gun was manufactured by the Marlin Firearms company for the United Defense Supply Company. It was designed and patented by Carl Swebilius founder of the High Standard Firearms company, who in turn had the Marlin Firearms company actually produce the gun for the UDS company. It was actually designed/developed in 1941 but was not actually produced until 1942, hence the later model number of "UD-42". They were originally intended as a substitute for the Thompson, but by the time they actually got into production, the Thompson/Auto-Ordnance company caught up with their back orders and the intended orders for the UD-42 SMG never materialized in any sizeable quantity. It is estimated that total production was approximately 15,000 with a large quantity being used in the Far East, mainly the Philippines. Some saw use with partisan groups in Europe.
They function in a blow-back type operation that uses a 20 round Thompson magazine. There are two unique features of this weapon, one is that it does have a hold-open catch, that holds the bolt open after the last round is fired. The other is that the right side of the weapon has a sliding cocking handle almost identical to the cocking handle on the FN-FAL rifles. It has a fixed front sight with a very interesting rear sight that actually sits on top of a shaft that raise out of the rear of the receiver, which is fully adjustable for windage and elevation. They were a very robust weapon being machined entirely out of steel.