Interesting. I have the EXACT same failure on the PTO side as we'll except mine also has some detonation along the top of my piston, on the exhaust side. I have a piston almost identical to your first picture.
Riding at boulder , started it up and drove it out of trailer, shut it off and got geared up to go and had a hard time starting it but got it started with new plugs and warmed it up for 4 or 5 mins, temp gauage was reading 20 c sled has no thermostat. Basically idled it over to show my pass and drove slowly up over the bridge and it died at first corner.
One person says deto another thinks cold seize, no pinging or knocking. Sensor never picked anything up. Bad fuel? Running chevron 94 with gallon of c12.
How cold was it that morning Tristan? Did you pin it when you left the lot? O rings and base gasket look good when you tore it apart? By the pics hard to tell if front or back of piston or both?
Damage was all around the whole piston which lead me to believe it was cold seize. No damage to the top of piston. There was alot of snow on the sled from the day before which makes me think that the sled never had enough time to warm up. The top end is at big johns now and he thinks bad fuel along with deto.
Looks like a pinch to me, my guess is cold seize. If it was while riding IO would say maybe snow ingestion but on the trail unless you had a bunch of ice or water in the intake somehow looking like a care of the cold seize to me