Well, best I can tell looking from the pics......
I'm screwed.
Looking down thru the holes with the prechambers removed, you can see a nice flat surface on pistons 2 thru 5.
On # 1, there are two big divots, almost the size of a quarter and somewhere between 1/16 to 1/8 inch deep, missing out of the top of the piston. When you first look straight down the hole, it's like looking at the center section of an hourglass, and if you look to either side of the "hourglass", you can see the two big irregular-shaped divots missing.
And no, the missing chunks are not inside the cylinder.
That would explain why the intake manifold, intake valves, injectors, and prechambers were so bloody clean on an engine with 215K on it - sometime previously, someone lunched #1 piston, pulled the head to get the chunks out, and then slapped it back together.