The rifle has been FTR'd at some point, not sure who by.
The "N" with a number, "M" with a number, "S" with a number are the codes for the contractors who made the various parts. S codes are for plants in the South of England, M for the Midlands (up around Birmingham area) the the N for anything North of that, including Scotland. There exists a COMPLETE list of contractors; it is several very large pages of very small type.
The Mark 2 rear sight (which is what you have) is one of the half-dozen or so rear sights for the Number 4 Rifle, ALL of which are correct for the RIFLE but may not be correct for a specific PERIOD o FACTORY. These range from the beautifully-made Mark 1 sight with the micrometer-screw adjustment (which was the biggest single bottleneck in production) to the lowly and often-despised Mark 2 (which you have). There also exist Mark 3 and Mark 4 BRITISH rear sights assembled from stamped parts (the Mark 3 is rare, owing to small numbers made because it was fragile and too exposed) and at least two Canadian models. They all interchange with about 15 minutes of work. Nothng wrong with the little Mark 2; you can do good shooting with it if you read the instruction manual AND have a spike bayonet handy.
You can download a complete 1942 RIFLE manual for free if you flip over to milsurps.com. It's in the Military Knowledge Library in the Lee-Enfield section. Membership is free (as here at CGN) and they have a veritable treasure-trove of free information for free download.
Nice toy.
Hope this helps.
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