It appears to be a Kohler piston (your favorite) and something went amuck with the valve keepers and dropped the valve, But it would be easier to guess seeing exactly what you seen as you opened it up.....
Originally Posted by piston slapper
You never learn anything until you admit you don't know it all...
"Why go out preserved when you can go out beat up, worn out, sliding in sideways screaming, Holy Sh!t What A Ride" !!!