My guess? A shaving from doing a muff mod. It could have been in the muffler for an indeterminate period of time attached to the underside of a hole and then found it's way into the engine. I had a circlip ear tear up a piston/cylinder and it got both sides of the cylinder so it must have gone through the piston's window or around the transfers or something. At any rate, a piece of debris can move around in an engine and wreak havoc. Without forensic analysis, there's probably no way to be sure of its origin. (I know a guy that could probably compare that sample of debris with other parts of the saw...)