Bear in mind that Norton upgraded the metallurgy of the gearset about '70-'71. This was in response to gearbox failures due to the Commando's horsepower. I don't know the extent of the changes made inside, but 4th gear (layshaft) was certainly done. The improved gear will have a '6' on one side. I don't think any others are so marked.
Note: 4th gear is also final drive, meaning the lower gears all drive through it. This will become obvious if you tear a tooth off by launching hard or popping a wheelie. The resulting knock you will feel/hear will be noticeable in every gear. If you are lucky, you only have to replace the one gear in this situation. Sometimes both. Really unlucky, you replace everything that rotates, along with the main case and possibly the inner case!
BTW, I've only seen three catastrophic failure modes: #1 Wheelies! 2-sudden engine destruction, 3-push starting after dropping the bike on the swung out kickstarter (sheers off the layshaft inside the box)
The external change made was to replace the clutch discs with ones designed to engage softly, to lessen the shock loading of the gearset, especially from first gear abuse. re:wheelies. These are the 4 plate solid fibre discs, later changed to the 5-plate bronze setup.