I believe both RP and Winchester/Western used A as their target wadcutter brass. I understand it was somewhat thinner to the first cannelure to assist seating and prevent deforming. Most older target shooters preferred this as Bullseye brass. IME, it might only last 19 firings instead of 20 so no real life difference.
C or D might be Federal's wadcutter brass, no difference from RP/WW other than a single cannelure.

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Front sight and squeeze