My cylinders are stamped "B". They also have the letters "xp" painted on the left corner from where the "B" is stamped. Is that "xp" an original marking or has this engine been messed with.

Looking in the parts book they list, "B", "C" & "D" pistons. What are the differences between these pistons. I know I need to get a "B" piston but I want to know what the differences are for my own knowledge.

Thanks.