Hollander goes by casting number rather than part number because salvage dealers went by casting numbers rather than by part number. However casting numbers are not indexed. The casting number you cite does not come up to a Chev or GMC 4 speed in my 1966 edition of my Hollander. A 48-54 SM 420 will hve a casting number of 591907. If it is in a half ton, it will be a torque tube. a 3/4 and 1 ton uses the same case with a half closed drive shaft. a 1 1/2 ton uses the same case but is open drive shaft. A SM 420 with a casting 3837332 would be a 55 and later with an open drive shaft.