Why does the 1/2 ton truck rear end need this but car rear end does not?
Part is installed on pinion gear shaft, in front of rear bearing snap ring.
What is its purpose?

1940-1954 Chevrolet Pickup NOS pinion flange retainer, part #3653622
eBay item --- Photo

1929 - 1957 Chevrolet Master Parts & Accessories Catalog
5.468 Retainer, Pinion Flange Packing
40-42 Comm.
46-54 1/2 ton part #3653622

1954 truck rear end General Description & illustration --Snap ring & Retainer are there but not pointed out. They are in front of arrow #4.
1949-1953 Passenger car rear end illustration--Snap ring, NO Retainer ---- General Description
Passenger car drivetrain cutaway (Scroll down) It shows 5.489 Ring (Snap Ring) but not retainer.

1954 truck shop manual, Under "Assembly[/b]" 1/2 ton axle
Number 2. Thread [b]pinion bearing oil retainer.......

1949-1953 Passenger car shop manual, Under "[b]Assembly[/b]" axle
NO oil retainer mentioned.