Ray,
I don't know much compared to others but I agree with you on the RM / RL / RS stuff. When I bought my car I found the "Encyclopedia of American Auto Manufacturing 1940-1974" (or something like that) and did a little digging. I was horrified by the idea that I might buy a clone so I went right away to the Plymouth section.
Anyway, if my memory is correct, there were very few Belvedere 2dr coupes made in 1969. It was somewhere around 9,000 give or take. According to the book, they were RL21 code cars.
Pretty interesting I think, and it basically convinced me my car wasn't a clone. My car was painted in the 1970's as well as receiving the engine it has (cop car 440). I thought to myself, who in 1975 would bother to paint and clone a 1969 piece of junk Belvedere? Plus, the K-frame, tranny, and leaf springs would be all wrong because the Belvedere would just have regular passenger car components. My fender tag and dashboard match properly so I wasn't losing too much sleep over it, I just found it to be interesting.
Why I'm telling you guys all of this I'm not really sure. Probably because this is the only group of people that would have remote interest. And, I've been waiting to share it for a few years.
Wouldn't you know it however that within a week some idiot looked at the car and said it was a "fake" because of the post. Do some homework, dork.