I’m at a loss; have 50 years experience and never seen this before. 1969 FB 350, stock 030 rebuild. Stock 47 heads. Summit k-2800 cam. Holley 0-8555s brand new carb. Pertronix kit. Was completing final drivability and LOUD clacking begins. Was pulling steady 20” vacuum. Started idling rough, louder and louder. Shut it down and checked compression; 3 of 8 had compression <100 psi. Looked at plugs (see pix) never seen this type of damage before. Ceramic gone, electrode bent over. I’m in big trouble now, started tearing it apart. Heads are a disaster, (see pix) 3 pistons ( not on same side) heavily damaged and pockmarked. Carb was never off so no way something got into engine. Look at this damage! Taking back to rebuilder tomorrow AM. Pilledcpan, shiny metal everywhere. Timing gears check out fine. Opinions?

show a picture of all the spark plugs.
Looks like the ceramic came out the plug and got bounced around in the cylinders.
Or something got sucked in the carb somehow.

Other 5 were normal; tan appearance with gap of .040 which is the increased gap needed for a Pertronix.

Who put the plugs in?

Not that this has anything with the failure, but stamping anything in the deck of a block is no good, especially where the fire ring for the head gasket sits.