From what I can tell in the cylinder/piston, I think the piston probably seized and then came loose.
Maybe high rpms for too long? Maybe forgot to mix oil in the gas?
I don't know how long the piston was slapping around (I certainly didn't do it any favors with the sunday ride), so anything is just a guess.
Can't feel any up/down play in the crank or the flywheel, so after some more fettling I think the cylinder is salvageable - at least for one more piston. The ring ridge I was worried about was just some carbon and the bore is pretty round and mostly in spec.
I ended up ordering a vertex piston, so we'll see how it goes when that arrives. Even if it's "good" - I think I'll probably replace it soon-ish.
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