Aussie four speed info.
If a cluster number ends in
496 it is a M20 (3.05:1 low gear ratio)
497 it is a M21 (2.54:1 low gear ratio)
484 it is a M21 LH SLR5000 L34 option and 1973 XU-1 6cyl homologated box.(2.32:1 low gear ratio)
694 it is a M22 (3.74:1 low gear ratio)
If a 6cyl input shaft ends in
923 it is a M20 (3.05:1 low gear ratio)
921 it is a M22 (3.74:1 low gear ratio)
If a V8 input shaft ends in
925 its a M20
924 its a M21.
If third gear ends in
491 its a M20
492 its a M21
696 its a M22
V8 4 spd M21 input shafts Neoprene type
One genuine shaft has 1 groove and one has 2 grooves.
So this shows that even on genuine parts the 1 groove 2 groove ID method is not reliable for M21 input shafts.