This is referred to as an AMC Gearbox. AMC means Associated Motor Cycles. AMC was the owners of AJS, Matchless, Francis-Barnett & James motorcycles and also purchased Norton in 1953. At the AMC factory in Woolwich near London. AMC developed a much improved version of the Norton gearbox and this was later to be used in all bigger models of AJS, Matchless and Norton. In the end only Norton survived and the same gearbox was used in the Norton Commando models.
Now, my gearbox is a replica AMC gearbox, built by the company TT Industries ( Trans Tasman Industries ) on New Zealand. In my case, the housings are made in magnesium alloy and the gear ratios are as follows:
1st 2nd 3rd 4th 5th 6th
2.294 : 1 1.755 : 1 1.418 : 1 1.243 : 1 1.1 : 1 1 : 1
Yes, it is a 6 speed gearbox with standard gear ratios since I figured that it will be easier to get away with a lower ratio on especially the first gear. The shafts will accept the standard Norton Commando clutch and sprockets.