What is the difference between a muncie m21 and m22?

the gear ratio is the only difference. In the M20 and M21 the gears were cut with more of a slope or angle to them, while M22 gears where "straight cut" and having almost no slope to them at all. This is why the M22 has it's distinctive whine when under load. The cases are the same but its what's inside that counts !