That's a great website. I am still trying to 'cipher which rear diff I have in my '59. It is in need of new parts (seals, gears, etc) and I need to measure everything so I can figure it out. It seems to have had a previous catastrophic failure before I picked up the truck 20 years ago. We found a piece of (ring?)gear tooth down inside the right axle tube. I am trying to locate the proper oil seal for the axle. Old one had a National #5801. Need a cross-reference guide for National seals and bearings