Originally Posted by wogpotter
Lee-Enfield magazines aren't "interchangeable" in the modern sense. However #4 & #5 magazines are the same magazine, but they do need fitting to the rifle individually. This is usually a question of adjusting the feed lips.
#1 (SMLE) magazines do not fit either #4, or #5 rifles & #4/#5 magazines do not fit #1 SMLE rifles.
While the Lee Enfield has a removable magazine it was never intended to be removed from the rifle for other than cleaning/maintenance.
The No4 and No5 mags are the same magazine. They well need fitting to individual rifles. You can get the No1 mags to physically fit a No4/5 rifle it well not feed properly.
The easy way to tell the difference between a No1 and the no4/5 mags is the two ribs pressed into the sides of the mag body. On the No1 mag the ribs go completely to the bottom of the mag. On the No4/5 it stops short of the bottom of the mag body, like this pic.