FH, Welcome to the forum.

Your rifle has been serviced, possibly by a field repair unit. The magazine is a replacement from a rifle that was more than likely beyond repair. The old s/n was struck through and yours applied. Why? Because that magazine worked with your rifle and needed to stay linked to it. Possibly when the s/n was restruck on the LHS of the butt socket also.

A good L-E primer is Stratton's Britishicon Enfield Riflesicon Vol. 2. Usually copies available on the different arms web sites. Check North Cape Publications, also.

Brad