Don't know if folks have seen their video showing the head - https://www.youtube.com/watch?v=aUBPPQxHa6M
I'm an engine internals newb, but it looks like there was valve/piston contact on two of the other cylinders (valve seat fragment - valve shaped ding on piston), apart from the one that went boom. Timing belt "issues" were my first thought.
10X+ Loser - Delinquent Racing - '86 TRUMPACO Merkur - '17 Sears Pointless, '16 Pointless, Vodden, Arse-Sweat, Button Turrible, Arse-Freeze - 2015 Vodden, Arse Freeze. '96 Ford Taurus SHO Skunk - '17 Vodden (Hope for the Future award - not me, the rookies), '15 Pointless. Button Turrible.