I came acorss a transmission that a guy rebuilt that he said it is an m-21. I took some random numbers off the core and my findings say the core was an m-20. Can you put a m-21 gearing in a m 20 case?

here are the random numbers i pulled off

3885010

3831731

2335508

v664

b111499

any help qould be appreciated thanks

The case is the same for M20, M21 and M22, so your answer is yes.