wiseco pistons used to have a p2, p4, p6, p8 as the last two numbers. it should be smack dab in the middle of the piston crown. the first 3 numbers were either 534 which was 85-86 lt250r, 552 which is 87, or 571 which was 88-92. The letter/number correspoded to the ammount of overbore (ie. pS=stock, p2=.020 over, p4=.040 over, p6= .060 over, p8=.080 over) ive only ever dealt with wiseco pistons so idk what other piston manufacturers use, or if wiseco still uses this number setup or not. you could always get a dial caliper or micrometer and measure the OD of the piston.