I can't tell from the picture but most of these have two nuts with a tabbed washer of some sort in between. Perhaps that's already been removed. Any how when putting a two-nut assembly back together be careful not to tighten the inner nut too tight because when you cinch down the loc nut it will take up the slack in the threads of the inner nut and the bearing preload will be too great. There should be instructions in the Timken or Eaton book Or maybe your GMC book describing the procedure to set proper preload. Too loose is just as bad as too tight. Just right is golden. :-)

You can get the numbers for the bearings right off the bearings, same with the seals unless the number is inked on the outer circumference. Measure the hub diameter at the sealing surface and the bore into which the seal fits and go to a bearing and seal catalogue to get current numbers if you can't cross reference. The Timken catalogue will have what you need but you may have to buy from one of the bearing houses as Timken doesn't sell direct so far as I know. Avoiding NAPA will save you a lot of money on seals and bearings.

1951 3800 1-ton
"Earning its keep from the get-go"
In the DITY Gallery
1962 261 (w/cam, Fenton headers, 2 carbs, MSD ign.), SM420 & Brown-Lipe 6231A 3spd aux. trans, stock axles & brakes. Owned since 1971.