I've never seen that label applied to a Slant6 car.
A few minutes on Google - out of curiosity myself - and it's not really clear, even on Allpar:
1. One site says 318 or 360 on '79 RR
2. 3 sites say RR was just a trim package.
3. 3 sites show the 225 as standard, but none specifically say NA RR.
So my guess is it may have been possible, but without a VIN character or Broadcast Sheet, hard to prove.
Maybe it's mentioned in the new car brochure ?
There was a 2-bbl option for the 225, with larger exhaust; didn't know that.
Either way, it seems odd, and Seller would be wise to have documentation; order form, window sticker, or broadcast sheet.