The final standard pattern of the venerable Lee Enfield as a standard-issue service rifle was the No4 MkII, introduced after the end of World War Two. The new pattern was adopted to resolve problems that had come about because of wartime simplifications to the rifle. Specifically, the use of kiln-dried wood for stocks. On the original No4 design, the trigger was pinned to the trigger guard, which was in turn connected to the front handguard. The sear was pinned to the receiver. If the handguard swelled, shrank, or warped (which was much more likely on the quickly kiln-dried stocks used during the war, compared to the naturally dried pre-war wood), the trigger/sear interaction could be impacted.

To solve this, the receiver was changed slightly to include a boss for attaching the trigger. This kept both the trigger and sear pinned to the (metal) receiver, and stock shifting was not longer a problem. In addition, a change was also made to remove the boss originally intended for mounting the magazine cutoff lever, so that No4 rifles would be made using No5 receiver forgings (this change did not result in a new rifle designation).

Many existing rifles were refitted to the new standard, with upgraded MkI rifles becoming Mk I/2 and upgraded MkI* rifles becoming MkI/3.