I think it is a hard concept to voice is the problem. A really big change in the middle of 1982 production was the helical gears changed from 15 to 17.5 degrees and it is hard to see this with the naked eye. It is not the angle you first notice when you see these are helical gears but it is the shape of the teeth.
It is hard to impossible to find the 15* gears if you need them. I think only the 5th gear is readily available. That's the one on the output shaft that riders are changing all the time to a higher ratio (after market part). So people have installed a 17.5* gear not knowing better or not caring. I have seen it said that extra noise was the consequence implying this was the only consequence, and it should be noted this is not the only consequence.
The 17.5* gears are supposed to be marked on one side with an X. Many reports from many sources of 17.5* gears that were not marked. So it behoves you to pay attention and if you find a gear with out an X know where it came from.
Sometimes the only solution is to replace all three helical gears and since the one on the middle shaft is only available with a new complete lay shaft the price really starts to add up.
I do wish there was a readily available gauge to measure these teeth with. Maybe there is. I've never heard of it's being used. There are gauges for measuring screw teeth but I think that is something a little different.
BTW, the last gear on the output shaft is first gear but the gear next to it with dogs on both sides is 4th gear.