During cruise at 600 ft AGL a bang was heard & acft experienced a vibration. When turning toward a fld to land a second bang was heard & vibration ceased. However, an uncommanded rt yaw was experienced & the nose began to tuck under. Acft contacted terrain despite remedial action attempt by plt. Investigation revealed tail rotor ass'y separated from acft inflt. Lab exam revealed gear box attachment studs failed due to low cycle fatigue. Tail rotor blade, s/n TCT-7478, had aprx 60% of phenolic tip block missing & 3 aluminum rivets missing from titanium skin. No bonding material was visible on interior blade skin. Acft was operated in a salt water envir. On 4/17/82, AD 82-17-04 was issued req inspection of tail rotor ass'y specifically for tip block security, void & retention every 7 days. AD revised (AD 82-17-04,R1) req compliance not to exceed 25 hr intervals. On 9/3/84 revision 2 req inspection reverting back to 7 day interval. Operator had not received revision 2 & last inspection performed on 10/4/84 aprx 21 hrs prior to accident.