Exactly
I think that AMC gearbox fared pretty well, considering it was designed in the early '50s for a range of motors up to 600cc and maybe 40HP.
The box in the Ranger is really sweet, and is original to the bike - except I replaced 2nd gear. I assume its got about 20,000 miles on it, 3K by me.
The box in the Commando is nice, too, but the shifting action not quite as crisp as the Ranger. Not sure how many miles on it, as I got the bike as a basket case, and the box is not original - 209xxx serial on it. I assume a previous owner broke the original and stuck a '72 box in.
When those gearboxes first came out, they were usually fitted with 18-tooth sprockets for the hotter engines, and 20-tooth for the milder engines, with a range available from 15 to 21 teeth. (from 1956 AMC parts manual)
Circa 1960, the standard sprockets were 16 or 17 tooth (1960 parts manual)
N15CS came stock with a 17-tooth sprocket, while the N15CSR, Atlas and P11 had 19-tooth (1968 parts manual)
Early Commandos came with 19-tooth, later with 20 or 22, with a range available from 19 to 24.
There's a big difference on the stress to a gearbox when being driven by a 750 or 828 cc motor, then driving the rear wheel with a 22-tooth sprocket as opposed to a 17- or 19-tooth. The standard sprocket for an N15 was 17 tooth, being driven by a 7.5:1 750 motor. Just 4 years later, the same basic box is being driven by a 10.5:1 engine and driving a 19 tooth sprocket. The next year, the same basic box is being driven by an 8.5:1 828cc motor, and driving a 20 or 22 tooth sprocket.