The AMC gearbox is one of the most successful of it's type ever designed. It has found a home in everything from AJS singles to fast, racing Commandos. It has a simplicity and robustness that makes it a favorite in Tritons and other specials.
After Associated Motor Cycles took over Norton in 1953, the gearbox was taken on as a project to be set up for use in multiple applications. The four speed AMC box was used first in AJS and Matchless heavyweights in 1956. In late '56 the box made it's way in to the 1957 Dominator 88 and 99, ES2, International and Model 50. The box has a history that extends back to the earlier Burman four speed gearbox, and from there to the even earlier Sturmey Archer box. They are so similar that some parts are interchangeable across the board.
The design was so good that it received minimal changes until the end of Commando production. The kickstart spring design was changed in 1962 (64 for Norton). The markings on the top of the case have an M for Matchless or an N for Norton. After the spring mod this was changed to MA and NA.
The Commando gearbox was always stamped with the numerical portion on the engine number, and should match the frame as well. 750 models are within a 100000 to 200000 series, and 850 boxes are in the 300000 series. The casing on the Commando was also reduced at the upper mounting bosses to make installation easier. It was thinned and a separate spacer was fitted between the engine mounting cradle and the box. Leaving this out can distort the cradle, so be careful!
In 1968, at number 128646, the high gear pair was modified for strength and durability, and these gears must be a matched set. The tooth profile and number was altered and they will not mesh properly if they are mixed. This also raised the ratio of first, second and third gears. Later on the 850 Commando, the second gear pair was altered starting at number 306591 to enable the bike to pass tougher noise regulations. The test involved passing a noise meter in second gear at a certain speed, so the gearing was altered to allow this to happen at lower revs. Clever those Brits.
The parts availability for the Commando gearbox is excellent. You can actually purchase a complete box assembled and ready to install from the Norvil Motorcycle Co.
With this in mind, you can use your Norton as intended for years to come.