Join Date: Sep 2000
Location: Washington, Michigan USA
Only sure way to tell is to pull the side cover and count the teeth on the input gear; if it has 24 teeth, it's a '63-'65 M20, if it has 26 teeth, it's a '66-'74 M20, if it has 21 teeth, it's an M21. Yours is a 1966 or earlier from the date code, built on May 25th between '63 and '66. Highly unlikely that it's an M22 (they have 20-degree helix angles on the gear teeth instead of the normal 45 degrees).
Is there a VIN derivative (8 or 9 digits) stamped adjacent to the date code? That would tell what it was originally installed in.
'69 Z28 Fathom Green
[This message has been edited by JohnZ (edited 05-01-2001).]