Got the pistons/rods out of the old motor last night. Here's a look at the piston from cyl1:
Rings seem smooth but the crown (?) of the piston obviously has some damage around it. The marks on the piston line up with the marks on the cylinder wall, so that piece fo spark plug obviously got stuck alongside the edge of the piston/wall, did some damage and then either disintegrated or flew out the exhaust.