My thanks to Lucas Jefferey who has provided some more information that has allowed this post to be updated.
During the Second World War the Australian Army started introducing canvas muzzle covers for its rifles to prevent debris falling down the barrel when they were not being used, possibly as early as 1942. These were clearly inspired by US practice where similar covers had been used on the M1 Garand for a number of years and were made of offcuts of herringbone twill and secured with short tapes:
After the war the Australian covers were redesigned and the pattern chosen was very simple, being a small green bag made of heavy duty canvas:
A set of male and female brass press studs are fitted to one corner of the mouth of the bag:
The cover is placed over the end of the rifle and then wrapped around to allow the studs to meet and secure it firmly in place:
The cover will fit both the SMLE and the No 4 rifles. The front of the cover is printed with the D/|\D mark indicating Australian Army ownership, the type of rifle it is intended to be used with (No1 Mk III), together with a date of 1952 and the manufacturer’s initials ‘MTX’:
MTX were Melbourne Textiles who had several large government clothing contracts during World War II and after. These muzzle covers seem to have been produced in some quantity but rarely used so mint examples like this are still easily available on the surplus market and are an interesting accessory to have with your Lee Enfield rifle.