Good AM NCRC51 and congrats on your latest purchase. While there are different rear seat part numbers for the SP1 and SP2, the fitment is the same. The only difference, from what I've been able to tell, are the silver brackets (SP1 appears to be made of steel while the SP1 is aluminum). I've owned both part numbers and have installed each on my '05 RC51. Here are some pics of the two different part numbers:
SP1 (77300-MCF-000) *part discontinued from Honda
View attachment 135539
SP2 (77300-MCF-D30) *part still available from Honda
View attachment 135540
Hope this helps,
Jason