I reomved the carbs and disassembled them. Found that the jets were clean and clear and the floats all worked the same. No difference noted from one carb to the next in respect to air fuel flow. Oil injection appeared to be working fine. Never got an overheat warning or lost water pressure at any time. No air leaks were found around the carbs or intake manifold. No idea what the timing was prior to the failure, but thing ran like a scallded dog until it failed. The quality of the fuel has been called into question, but with no other damage on any other pistons, that's a hard pill for me to swallow (even though plausible). Spark plugs are the factory NGK recommended and were changed during the spring service.
what did the plug look like? You got a pic of it? If it was run lean it would be vey white/ashy. That damage kinda looks like the bits of the ring smashing around. There were two rings n that piston right?