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Original 1951 3100 1/2 ton axles Do Not have a pinion seal.
Original 1951 3100 1/2 ton trucks came with a propeller shaft inside a torque tube, which is also called a enclosed driveline or driveshaft.
There is only one u-joint inside a torque tube, located at the end of the transmission.
The pinion splines are inserted into the propeller shaft and is dowel pinned in place. Picture

Torque Tube parts illlustration

3/4 ton trucks and up all had open driveshafts
1-1/2 Ton Rear Axle parts illustration

Someone must of replaced the original enclosed drive line with a open driveshaft drive or (Hotchkiss Drive)
Open driveshaft carriers do have a pinion oil seal at the rear u-joint axle connection.

NK1011 and 273 do not show in the 1950 Chevrolet Restoration Pack Go to pdf page 6 for rear axle numbers