As in the customer or the dealer?
It was up to the customer - he could order M20, M21 or M22. Although the 3.73 was the "standard" Z/28 axle if no other ratio was ordered, Chevrolet's AMA submission for the Z/28 option shows availability of the 3.55 as an "economy" option, the 4.10 as a "Performance" option, and the 3.07 as a "Special Order" option.
The 3.55, 3.73, and 4.10 were available with either M20, M21, or M22, but the 3.07 "Special Order" axle was only available with M20 (for obvious reasons).
Forgive me for following up on such on old post, but I thought it easier to show the thread history, than restate it all. So here is my question:
If I have an original 1968 “MW” coded L35 with an original transmission, was it possible to order this car with an M21 from the factory?
Also, is there any way to determine an M20 or M21 by just looking?!? Or perhaps a stamp somewhere? Thanks?