Is the damage on the piston on both faces or only one? I can't tell from your photos. Assuming it is on both faces:
I didn't see this kind of damage occurring unless the ring broke and ended up between the piston and cylinder wall.
So, I did some research and here is what I found (this from www.ms-motor-service.com):
Initially, it looks like this:
But in these cases, damage above the skirt should be minimal. So, I did some more research and found this:
It definitely sounds like no end gap could cause this, although it is not identified specifically in the article. Your piston looks like image #4.
However, those who believe in the Motoman break-in strategy might want to take a good look at this....
If anyone wants a little light reading on this: http://www.boosttown.com/engine/piston_damage.pdf