Without a full tear down it looks like it got very hot and began a melt down. Meaning a lean series of runs. The possibility also exists that a bearing could have also let go prior to the piston/ring problen, causing a piece of the bearing to slide up the side of the piston and initiating the problem, but the melted condition of the piston leans more towards either too little oil, too lean a mixture setting for prolonged periods of time, or both. The amount of oil sludge pretty much lowers the chance of too little oil to almost zero.

That's the way I see it, but others may have a significantly different view.