Looks to me like its been stamped if not internal FOD maybe external FOD. Definatly not detonation damage.

In my work we seen large 15 litre heavy duty engines swallow all four tulip valves, crunch them up and spit it out thru the turbo. Chances are that you may never know what the object was that did the stamping of the piston and head if it was not internal, as it probably left via the exhaust

Bad Tooth, did you have a look at the small end of the rod? I had a piece come off the small end once and stick the piston to the top of the head.

Richard