Good picture.
Looks to me like theres been a serious case of piston crown overheating, possibly detonation (pinking).
If the top piston ring had boken this would have possibly been due to poor assembly by a previous repairer, or a cold siezure due to too higher engine (piston) speed when cold.
The previous description was of damage at the 5'oclock position, I don't think anything was mentioned about it being right above the piston ring land.
One other possible cause would be if the top piston ring had become seriously gummed up or stuck (same thing) this would cause the ring to overheat and the ring gap would start to wear a line in the cyclinder bore...difficult to diagnose as the ring is well and truly stuck now by the llok of things.
