I'm unsure, is your service manual any help.

I'd take a guess that the brass screw/plug is to oil the bearings with the same oil as in the rear axle, newer axles don't have that provision. the oil should get there on a hillside or fast turn, always oil them or some grease at assembly though. I would not pack the whole thing full of grease just as I never do on front hubs.

Grigg

•1951 GMC 250 in the Project Journals
•1948 Chevrolet 6400 - Detroit Diesel 4-53T - Roadranger 10 speed overdrive - 4 wheel disc brakes
•1952 Chevrolet 3800 pickup
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"First, get a clear notion of what you desire to accomplish, and then in all probability you will succeed in doing it..." -Henry Maudslay-