M20 Wide ratio standard duty
M21 Close ratio standard duty
M22 Close ratio heavy duty
The parts mix and match quite a bit and you really need to pull the cover off and look at the gears to see what you have. The M22 has a different angle on the gears that cause the distinctive sound. Even with the different angle its hard to see the difference if you don't know what your looking at.
I have had all three in my car at various times. I ran a Richmond 5 speed for many years. They are way more fun but hard on parts at the race track. I switched to an auto more for racing. A street car needs a stick.