'06 RM250 Piston Size

Does anyone happen to know how to identify which piston is supposed to be used when replacing a top end in an '06 RM250? The parts catalog has two pistons listed. Part numbers are...12110-37F20-0F0 & 12110-37F30-0F0. I know Yamaha's have A,B,C & D size cylinders which have the letter stamped on the cylinder. I can't find any such reference on the RM.

J32, yes the bike is stock. This is the first top end for this bike. I've always run OEM pistons and I don't have any experience with the WISECO kits. Do you have good results with WISECO pistons? I race hare scrambles and never had a failure with OEM parts (knock on wood).

Yellow, that's what I needed to know, thanks! Just looking in the parts fiche, you can't tell. I assume the rings are 12141-37F00 for 05-06 and 12141-37E02 for the 02-04 as well...correct?