Piston slap was quite common in older motorbike and older aero engines. It used to be very common in police triumphs sold on to the public. It was the sign of being well used. It tended to reduce as the engine warmed up. Ring slap is when the ring journals are a bit worn so the ring isn't stable in the piston. This was very rare and I can't really see the cause unless the bore is so worn that the ring wobbles. It could be because the wrong oversize ring was fitted after a rebore.