Since you've already confirmed it's not an M-22, if you want to determine if it's an M-20, or M-21, simply count the teeth on the input shaft. 21 teeth = M-20, 26 teeth = M-21/22. The exception is the Early M-20 with 24 teeth.
(M-21, and M-22 have the same gear ratios).
ALL M-22 regardless of the year should have a drain plug.
ALL 70 up "661" cases have the drain plug regardless of ratio.
ALL "661" boxes from 70 - 74 have 26 spline input shaft, and 32 spline output shaft.
ALl info taken from
www.gearzone.net