The SMLE had 4 types of mags:
Type 1 had a link loop at the front to attach it to the rifle just as the earlier Long Lees, none of the parts were numbered.
Type 2 was made deeper at the front & did away with the link loop, the auxiliary spring at the front was also changed, the auxiliary spring & case were stamped with a 2.
Type 3 was a conversion of the earlier types for Mk.VII ammo, the case was modified by removing the left feed lip & a new lip riveted in place, the follower was also modified & a small spring steel bit was riveted to the bottom of the rear spine, the auxiliary spring was also changed again, the case & auxiliary spring were stamped with a 3 & the follower with a 2.
Types 1, 2 & 3 all have the rotating rh feed lip.
Type 4 is the standard mag we see on SMLEs, the case was changed, the follower was changed & so was the auxiliary spring, case & spring marked with a 4 & the follower with a 3.
Type 1 & 2 are for use with Mk.VI ammo.
Types 3 & 4 for use with Mk.VI & Mk.VII ammo
Type 1 was issued with the Mk.1 SMLE, the type 2 was introduced with the Mk.1* SMLE in 1906 and was also used in early Mk.IIIs until the introduction of Mk.VII ammo. Types 3 & 4 was used after that, you will find nearly all SMLEs have the relief cut in the side of the trigger guard & inside the mag well cut into the forend to accommodate the rotating rh feed lip.
Bit of a warning, do not attempt fitting these mags to a rifle without the relief, getting them in is not a problem, getting them out is another story!!
Thank you to Stuart Wilson for the article and photographs