You are 100% correct - Mopar Rallye Green is the same Ditzler formula as Chevy Rallye Green, even though they have different formula numbers. I researched this (through PPG)when painting my former 69 Road Runner convertible (see attached photo). I have had many "discussions" over the years regarding this. By the way, my car was coded 99, along with the Y39 special order code, and had the same "SP Green" written on the brace behind the rear seat, as yours did.
Not 100% but working on it. So what does the "SP" stand for? Spring or Special?
I have no pics of an F6 RR but many Plymouth-spring green. I did a spray out yesterday of the 1969 Plymouth # 99 "spring green metallic" -- PPG paint code # 44032 and it sent me looking at the color charts for another color.
Do you have a pic of your "sp green" writing?
Unfortunately I don't have a photo of the writing. I didn't take a photo of it because I thought it was done by a previous owner! Realized it wasn't, when I saw your photo a while back!
