Thats the only way it was available all three years. Small block SS's could only get the M20, L34/L35s could get the M20 or M21, Z28's and L78/L89s could get M20, M21, M22. (except in 67 when I think the L78 could only be had with an M21). Nothing else got the Muncie 4 speeds of any kind, except COPO cars. Muncies were even an option on SS's, base SS's got a HD 3 speed if you didn't select a 4 speed. Z28's required a 4 speed, the rest got Saginaw 3 and 4 speeds if a manual transmission was selected.