First guess secondary cam chain tensioner shim. Second guess cam bucket shim, but two of those side by side are needed to be the same diameter as a penny. I don't see anything in the gearbox that looks like that but other guys here on the site may know more than me - looks like the wear around the outside indicates a spring or the end of a shaft that is hollow rubbing it, hence my first choice. Funny thing is guys joke about finding debris large enough to have part numbers on it on their magnetic plugs, but you actually did !
Not funny thing is it got beat up getting to the magnetic plug so wonder what if anything else got bent, damaged, etc. Any other metal on the magnetic plug ?