SMLE - Short (as opposed to "long" - from the days when an Infantry rifle was like 29" barrel); Magazine (it had one - from the days when many soldier's rifles were single shot); Lee - the American guy who designed the action / magazine system; Enfield - the British Arsenal that designed the rifling profile for that barrel. As I understand how the name was used, SMLE was to differentiate it from the "Long Lee" or the "Lee Metford" rifles that preceded it.
I am pretty sure that I read that circa 1920's, the British gave all their "in-service" guns new names - the SMLE became the No. 1, the .22 trainer version became the No. 2, the P14 became the No. 3, then with WWII came the No. 4 and the No. 5. I do not know what was the No. 6, but I think the No. 4 in .22 trainer was called the No. 7. I believe after WWII, they might have changed the names used, once more? - so eventually had the L39 and the L42 rifles, then FAL and others. Minor alterations to the major design would have resulted in "asterisks" or "star" - more significant alterations became "Marks" of the main design - so can get a No. 4 Mk1, a No. 4 Mk1*, or a No. 4 Mk2 - each are different to the others, in some way. When a No. 4 Mk.1* was converted to the Mark 2 modification, it became the Mk 1/3 - pronounced "Mark One Oblique Three" - the No. 4 Mk 1 when converted became the No. 4 Mk 1/2. At some point they swapped from using Roman numerals to Arabic numerals, but I do not know when that was done, or if that was done by all makers - for example, I think there was No. I Mk. III and No. 1 Mk. 3
The stamp on your rifle should be "GR" - for "George Rex" - "George the King" (of England) - the guy that actually owned the thing, originally.
