Mopar stock steel rims....

while others are complete steel solid rims ?

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Rims with solid centers with no gaps on the inner edge are for heavy duty applications, like some station wagons & B & C bodies with the A35 - Trailer towing package. These are all 15" rims & A-bodies were never available from the factory with 15s.

Here is one from my 1970 Fury III with A35;