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.
Driver w/ Delinquent Racing - 1986 Badger Badger Badger XR Merkur - 2016 Sears Pointless, Vodden, Arse-Sweat, Button Turrible, Arse-Freeze - 2015 Vodden (Worst Best Lap, #1 in Spec Merkur), Arse Freeze (We Beat The Stig, #2 in Spec Merkur). 1996 Ford Taurus SHO Skunk (now resting comfortably at the car spa) - 2015 Sears Pointless. Button Turrible.