I feel relieved for you. When you said 'metal particles', I envisioned shavings and slivers, basically big stuff. The much finer particles you're describing are, as you already think, probably the result of the rust and down time while traveling. Once everything wears in, and a couple more oil changes pass by, I think you'll have a clean magnet again. Maybe I should try to locate a magnetic drain plug for my 280, just to see how the M117 is holding together. But then again, with no prior history, and unknown actual mileage, maybe I don't want to know.
73 280 SEL 4.5