The metal parts on the bottom look like broken pieces of spring or spring seat.
I think the broken metal is most likely from the valve spring, based on the fact that it matches the valve spring's texture perfectly.
My plan is to use a borescope to inspect the piston. If I see damage inside the cylinder, I'll pull the entire engine. If I don't see any damage in the piston, I'll pull the head & re-inspect the piston again when the head is off.
