You did say the pieces you retrieved were Aluminum. If they are non ferrous, a magnet won't attract them, and they would have to be steel if they were bearing spall. Very hard to figure from where they came, especially since the aluminum components you have shown do not appear to have tell-tale chunks missing. If there were to be no screen inside your muffler, that could allow something to get in thru the exhaust port, or if there are some pieces that got sucked thru your carb or somehow broke off from inside it, those could get into the combustion chamber thru the transfers. Air filter would generally protect from something external going walkabout in your saw. I'm not an expert either, this is a puzzle and I'm interested in hearing what you find out.