3837334 is the casting number for the case of an SM420 from '63-'67 series 10 thru 30 trucks. I think these later SM420's were a "second design". Your case casting date E246 would translate as May 24, 1966.
681701 is the casting/part number for the rear bearing retainer, though I don't find that number in my parts books. It looks like
this tranny may have that number on the rear retainer. I did find a 591701 for the retainer on the '48-'64 models, but that number should be stamped, not raised.
