The MCEM 3 was one of a group of different submachine gun designs tendered by several British small arms companies during the late 1940s for the contract to replace the venerable STEN gun. The MCEM 3 was designed by a team from the Royal Small Arms Factory at Enfield in 1947, along with two other rival designs also from Enfield.
The MCEM 3, or Machine Carbine, Experimental Model 3, was one of the more conventional designs with a tubular receiver similar to the STENs, a vertical magazine forward of the receiver and a standard blowback action. Relatively little can be found about it online but it is believed to have been chambered in 9mm with a slightly curved box magazine. As can be seen in the photographs above a Lee-Enfield No.4 bayonet could be fixed and it had an interesting socket-attaching rear stock with wooden furniture - the stock was inserted into the pistol grip and then twisted to lock into place. The weapon could be fired with or without the stock fitted. Unlike most contemporary submachine guns the MCEM 3 eschewed the now common pistol grip, instead opting for a more traditional rifle-style sloping grip - like those found on the STEN MkI, II, and III.
Other competing designs for the contract came from BSA with the Experimental Machine Carbine, 1949, several designs from other teams within Enfield including the more advanced MCEM 2, andthe MCEM 1 designed by Harold Turpin, co-designer of the STEN gun. Aswell as the ‘Patchett Machine Carbine' designed by George Patchett at the Sterling Armaments Company. It was the Patchett-designed Sterling that went on to win the contract, later entering service as the L2A1.
Image 1 & 2 Source
Image 3 & 4 Source