The poster is correct. The "R" prefix denotes a standard or "Regular" sized frame; the "E" prefix denotes factory installed Auto-ejectors.
The only other Smith serial number prefix stamps are "FW" which denotes Hunter's "Featherweight" or smaller sized frame.
There were no "R" frames produced during the Marlin production era so all Marlin era frames were stamped with the "FW" prefix stamp, but they also included another prefix stamp; an "S" which stamp distinguishes the late Marlin era Smith guns from those produced by the old L.C. Smith Gun Co. When Marlin reintroduced the Smith gun in 1968, those guns were on "FW" frames that also included a new code stamp; these late guns are serial number prefix stamped "FWM".
