I worked with a guy who was a pilot in Vietnam and he said torquing that thing down was the favorite part of the job. Apparently flying out there sucked, he was telling me the other guys would string zipline-like wire from tree to tree in clearings to catch tail rotors, all manner of shit, so if you weren't flying it was pretty nice
The one in the picture is in pretty good shape. It is just a retaining bolt. It is not holding the torque of the tail rotor, just preventing it/them (some helicopters have multiple paddles) from backing off the tail rotor drive shaft. The teeth on the nut are to prevent the jesus nut itself from backing off its bolt.