. Norton boxes have a prefix of "N" or "NA." Matchless boxes have a prefix of "M" or "MA." I believe the single prefixes were used post 1957 and the "A" was added to both after AMC bought Norton around 1962.
David
AMC actually bought Norton around 1954 (blame the featherbed), but later, moved Norton production to Plumstead around 1962. The AMC box was introduced in 1956, the change to 'A' suffix on the gearbox probably refers to the altered ratios brought in in 1960.
Starting from the 'horizontal' gearbox which actually shared its gearcluster with the 'vertical' box and even the pre-War 'doll's head' box, the early AMC had a closer first gear and much lower third. For the 'NA' box used a fractionally closer sleeve pair, a fractionally wider third pair than the pre-AMC giving the same 1.21:1 ratio as 'horizontal' and earlier, and closer second and first.
If you know your Norton boxes and tooth numbers, you can select your own gear cluster. But be careful - the AMC box needs a shoulder on the mainshaft first gear, and third main does not have clearance for the bronze thrust washer between third and sleeve gear on the earlier boxes, so engagement dogs do not line up if you mix box types with the third and fourth mainshaft gears. Confused?
Well, you could get the same result as an 'NA' box by taking a pre-AMC box and adding a late Commando second and AMC first.