IIRC they are different (mounting). Parts can be purchased individually for them so one can convert SP1 to SP2 or vice versa, but with used prices for complete units, it's usually cheaper to just buy the correct application outright.
I actually wrote off an RC51 because of headshake without a damper. My current RC51 now has one.
Thanks guys, yall have been a huge help and are an asset to this virgin RC rider. As a Retired WERA rider it almost makes me subconsciously need one. As I understand it, these RC51's have crap suspension and are highside happy. I have had a few of those and HATE THEM!!! so anything to help i will happily invest in. I Havent put it through the paces yet to measure it but will do so soon. Wish me luck.