Denny,
the SM420 came out in late 47 I believe. They were a redesign of the old four speed with a synchro design. First is really low and second is lower than the first on the three speeds. The offroad rock climbers really love them because they have such low gearing and they are tough. I don't know how far up in the truck size they were used but certainly they were used on the 3/4s and 1 Tons. I believe that they were built by Muncie.
