This Rear bearing retainer uses the oil seal.
That rear bearing retainer was/is used on 3/4 ton and 1 ton open drive shaft trucks.

Here is a Picture of SM420 4 speed with a closed drive shaft (torque Tube) rear bearing retainer used on 1/2 ton trucks. No need for oil seal.

The link http://chevy.oldcarmanualproject.com/parts/1929_54/220.htm you provided is showing a open drive shaft rear bearing retainer.