Here is how it works:
No. 7 Mk. I/L Bayonet
The No. 7 Mk. I/L (number seven, mark one, land service) was intended to address a number of desires: 1) Replace the No. 4 spike bayonet (that nobody liked); 2) Utilize the bowie blade of the No. 5 Mk. I Jungle Carbine bayonet (that everybody liked); and, 3) Serve a dual role as a fighting knife. The No. 7 Mk. I/L was a very innovative and complex design. The No. 7 Mk. I/L effectively integrated the No. 4 socket and a synthetic composition grip, with the blade & crosspiece of the No. 5 bayonet. As a result, the No. 7 Mk. I/L would mount to the No. 4 rifle, the Mk. V Sten machine carbine, and the Sterling L2 submachine gun.
176,000 No. 7 Mk. I/L bayonets were produced. The design was perfected by the Wilkinson Sword Co., who produced 1,000 bayonets in 1944. Mass production was carried out by four manufacturers from 1945-1948. The four makers and their production are as follows: Birmingham Small Arms, Ltd. - 25,000; Elkington & Co. Ltd., Birmingham - 20,000; Royal Ordinance Factory, Poole – 30,000; and Royal Ordinance Factory, Newport – 100,000. Examples are found with both reddish-brown and black grips.
The No. 7 Mk. I/L bayonet shown below was made by Elkington & Co. and is marked with their dispersal code “M78”. Elkington was a legendary producer of fine silver plate. Elkington & Co. invented the electroplating process in the 1830s.
When the pommel is stowed, the No. 7 Mk. I/L looks like a conventional knife bayonet. In this configuration, it would mount to the Sterling L2 SMG. In order to mount to the No. 4 rifle (shown) or Mark V Sten, the pommel rotated 180 degrees to become a socket.
Despite all of it's ingenuity, the No. 7 Mk. I/L came to illustrate the old adage that a camel is a horse, as designed by committee. After an errant fired bullet struck the crosspiece during testing, the Ministry of Defense became concerned that the No. 7 Mk. I/L bayonet flexed too much when mounted to the No. 4 rifle. They ultimately decided only to issue it with the Sten Machine Carbine and Sterling L2 Sub Machine Gun. However, a few No. 7 Mk. I/L's were issued to units with the No. 4 rifle for ceremonial use.