[Steve Tinker]

That could have happened in the last 1,000 miles or 12,000 miles. Whatever caused the debris its not good news.

1) Could your engine have the later (green) chain guides - if so, thats possibly the problem, though the gude plastic is not a soft material.
2) Probably not the camshaft end plugs, if one or part was missing you would have oil leaking everywhere.
3) Valve guide seals would also leak oil into the combustion chamber and engine would smoke.
4) Good call on park tube O ring, have they recently been removed - check for possibe leaks around valve cover.

Maybe time to remove the sump and check for debris in the oil strainer and bottom of sump cover. There maybe other clues in there.