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I don't know much but that looks like chipping to me, not cutting. Then again I try not to use stone tools
Flushed it out a few times? Any evidence come out in the drain?
On my DR (which granted I knew to have metal bits in it) there were fragments hiding in the tiniest places, no way I could have found them without splitting the case. But you would think with the damage to that piston there would be some evidence elsewhere...
Good luck!