I am guessing you dropped a hardened valve seat out of that last head picture. That exhaust valve is sitting way deeper in it's seat than the other exhaust valve that is standing up more.
So the seat came out, broke up, and beat that hole of the head worse than the others. Chips backed up through the intake to the other cylinders.
As hard as those chips of steel are, sounds like an exhaust seat, they are made of hardened steel like that.
