Rearend Help for a 1955 2nd

[dcmalone]

The rearend casting numbers on the truck to be exact. While totally re-doing the rear brakes on my 1955 2nd I noticed a little bit of a drip from the pinion. In an attempt to find a replacement seal at Autozone I was asked if it was a HO33,HO35, HO 52, HO 72, Spicer 45, etc. Honestly I have no idea what the difference is and no idea what I need. It's a stock 10 bolt. I did however find the only numbers apparently on the center section:

3707340
D1

Does anyone have any idea what these mean or what it translates to in terms of HO33, etc? I have searched this and several other forums with no real luck. Any help would be appreciated.

[G&R's57GMC]

Check out http://www.fillingstation.com

[dcmalone]

Thanks for the link.