A possible guess based on experience from a C-150 which has the same
cylinders -
It could be a wrist pin end plug scrubbing the cylinder wall. This
was caused by a loose (i. e. floating) wrist pin rod bushing. A C-150
of ours always had produced a very small amount of aluminum which our
mechanic condemmed as having a lemon engine.

On teardown we found one wrist pin bushing loose in the rod end, &
that mechanical rectification was causing the wrist pin end to scrubb
slightly on the cylinder wall. I suspect the bushing had been cracked
loose during a previous disassembly.

We redid it with Millenium cylinders & a new rod of course. It has
been good ever since. It now is again at or beyond TBO.