Stop in for a cup of coffee

Hmmm. OK. Seen NOS about a million times, but never with the 'r' in it. What does that actually mean? Old piece that is not factory, but was available back then?
Yup older aftermarket
1960 61 62 63 64 65 66 Mopar A Body complete exhaust for 6 cyl | eBay
Good deal for me. Really just want the muffler and tailpipe but the price is right