As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PROPELLER WENT TO FEATHER IN FLIGHT AND WAS UNABLE TO UNFEATHER. AFTER RECEIVING THE GOVERNOR FOR INSPECTION IT WAS DISCOVERED THAT THE BOTTOM END OF THE PILOT VALVE (P/N 5221-220) HAD BROKEN OFF AND WAS MISSING. IT WAS PRESUMED THAT IT EXITED THE BOTTOM END OF THE DRIVE GEAR AND ENTERED THE ENGINE SUMP. THE OPERATOR WAS NOTIFIED AND THE MISSING PIECE WAS RETRIEVED.