head scratcher

[CrackFox]

i bought a second hand 5hp 4 stroke mariner, long story short i decided to do a full strip down, the decals on the box and the serial number all say its 5hp,made in belgium 2002 , but i pulled the piston out and its got a 6 on it? does anybody know if the top end is the only difference on 4,5,and 6hp mariner 4 strokes, , all i can think is somebody put a 6hp top end on a 5hp bottom end , i just wanna check im not making some rookie error, i need to order rings, and want to make sure i get the right ones, cheers