As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
NEW ENGINE AIR FILTERS WERE ORDERED FROM BEECH. WHILE INSTALLING A FILTER, IT WAS DISCOVERED THAT THE FRAME WAS ASSEMBLED WITH A PULLED 'POP' RIVET. THE RIVET STEM WAS NOT REMOVED FROM THE RIVET AFTER IT WAS PULLED. ONE STEM FELL TO THE FLOOR. FURTHER INVESTIGATION FOUND ONE STEM LODGED TO THE FILTER ELEMENT 'FINS' AND 2 OTHERS STILL IN RIVETS, BUT LOOSE.