Mike, while we wait to see the new pic of your casting numbers mentioned above, you should take another look at a different point on the transmission case. The pic below shows the location of the "stamped" transmission unit serial numbers on the SM420 4 speed, as indicated in the 1955 2nd Series Truck Shop Manual. Take a picture of yours (wire brushing/cleaning as needed of course) and post that picture too. Once we have both pictures to review,we should be able to best pinpoint what you have.
~ Dan 1951 Chevy 3 window 3100 Follow this story in the DITY Gallery "My Grandpa Carl's Truck and How it Became Mine" 1966 Chevelle (Wife's Hot Rod) | 2013 Chevy Silverado (Current daily driver) US Army MSG Retired (1977-1998) | Com Fac Maint Lead Tech Retired (1998-2021)
