For a start, the RC was the first Stradic GTM reel with the chamfered lip AR-C spool. There's also a change of bail arm/roller type somewhere between the RA and RC. I've no longer got Shimano catalogues prior to 2010, when the RC model first appeared, to be sure that there were no changes to bearing types, gear ratios, line lay, weights etc.
Shimano rarely roll over to a new model without at least the odd little tweak, even if they do sometimes look rather similar at first glance. It's also worth bearing in mind that, as there was an RB model in between RA and RC, an RA is likely to be t least 10-15 years old. IIRC the original Stradic GTM hung on until the very late 90s until being replaced by the RA. I know I bought an old stock original GTM in Italy in 2001.