I'm interested in this answer also. There had to be a reason and the fact these pieces are (a) only in one piston and (b) apparently were cast in place and symmetrically located gives me pause. At least I think they would have been cast in place. If they had been pressed in, the different thermal expansion properties might cause them to dislocate and shift with time, stress, use, etc and it wouldn't be good if even one of those came loose (unless they're part of a cast-in-place ring). Did you notice in which cylinder this piston was?
