What was found
PROPELLER BOLTS (AN6-57A) BROKE IN FLIGHT AT CRUISE POWER (2500 RPM) AT 7500 FT. MSL. NOTICED THAT ONE BOLT WAS MISSING AND THE NUT ON ANOTHER COULD BE REMOVED BY HAND. POSSIBLE THAT TWO NUTS, (MS 20365-624) LOOSENED CAUSING VIBRATION IN PROPELLER WHICH CAUSED BOLT FAILURE . NOTE: THAT ONLY THE TWO NUTS THAT SEEMED TO BE LOOSE, WERE REPLACED 124 HOURS BEFORE FAILURE. ALL OTHER NUTS WERE STILL INTACT. PROPELLER SEPARATED FROM ENGINE IN FLIGHT.