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SMT, you're not the first person to wonder about this. I usually look at the bottom of the post to see if there's a clue. Since I have an FLHRC (RK Classic) I usually abbreviate it as RK Cl, so folks will know. It can be confusing.
Technically, according to H-D.com, as you said, the RK Custom is FLHRS and the RK Classic is FLHRC (mine is an 06 and is called the FLHRCI, I reckon they decided to drop the "I", because they are all fuel "I"njected - were last year, too.
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