A quick answer to start things off.
R is the engine series, A is the body style, 20--22 is the model progression/update.
I believe the 20, 21 and 22 all have the same sloped front and use the same basic guard shape (are interchangeable) but each has a slightly different guard lip shape. I understand that the very last 22s (late 74) have the guard lip that gives the most clearance
A 23 model moves to the flat/longer front.
I'd go for the RA-22 guard if there are the three options of the fibrglass one.
Regards
Rodger
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