Marlin-Rockwell produced the Colt Model 1914 (the "Potatodigger") before WWI. They also produced the BAR during it's initial production run. They also built Model 1917 and model 1918 machineguns in many variations- tank, aircraft, anti-aircraft etc. They also made bombs in WWI.
In WWII they were contracted for many military items. M1 Carbine parts (barrels, stocks, hand guards and misc. small parts) As stated above they produced the never accepted UDM '42.
In Korean War era Marlin manufactured for the Ordnance Dept. M1 rifle barrels, stock assemblies for the M1 Carbine, barrels for the M3 submachinegun and other items.
The above information is from Marlin Firearms; A History of the Guns and the Company That Made Them by William Brophy