Depending on what year transmission you're dealing with, some have seals that are essentially the same as the output shaft seal or axle seal, only smaller, and some use O rings. I believe the change on the 3 speeds was either '56 or '57. In either case, you should be able to take one of the old seals to a local parts store and have it matched up. I don't thin I still have the numbers for them, but that's what I did when I went through the SM319 in mine.
