Difference Between Muncie M20 And M21

20081211m20 , m21 4 speed gear boxes how does one identify which is which im changing from auto to manual, ive been planing it for a bit and slowly buying all that i need , the box was purchased 6 or 7 months ago out of a vk 308 with very low kstoday my mate asked howw to tell thee diff between m20 m21, i didnt know so if some one out there could tell me how to identify one from the.