My thoughts are pretty much along with Gas-Holes.
Detonation was likely an issue throughout the engine, as every cylinder had substantial amounts of carbon, like Patch said, will cause detonation as it acts like a builtin heater in each cylinder that it's present.
HOWEVER since this was really the only piston that had damage, SOMETHING had to have gotten in there. As Gas-Hole said, those are DENTS not fractures.
The likely hood of something getting in there is 50/50. It could have been part of a spark plug, or it could have been the piece of the piston that broke loose, and is either missing, or is stuck in the ring grove below what is visible. The oil ring could have come apart, but since there are no signs of scarring in the cylinder wall, the mysterious solid object that caused the dents most likely was either the piece of the piston itself (random oops from factory when piston was installed, or fractured at some point later) or some piece of the spark plug.
Will pull the piston out today, carefully, and look for anything stuck in the top ring groove ABOVE the top ring. If nothing there, then I would have to guess more than 50/50 the mysterious object was the piece of the piston itself.
This is fun don't you think?