SM420 is the 4 spd transmission that was used in the 1/2 ton thru 2 1/2 ton trucks from 1947 thru 1967. It is a desireable tranny for the off-road community due to the 7-1 granny low first gear. I think that it is just about the lowest 1st gear tranny ever built for a production truck. Hope this helps.
Garry K in Port Orchard, WA
