I'm with Ethan on this. It also depends on if the
shackle was tightened enough that the threads are buried into the female side of the
shackle and that the bow was compressed. I've looked at thousands of shackles that are part of rigging systems installed by us and other companies. The ones that are tightened with a properly sized wrench don't seem to come loose. Finger tightened shackles often are backing out. I have our guys wrench tighten all of our shackles.
What I saw was a nylon tie on the other shackles. Possibly the nylon decomposed and fell off. Maybe there was a crack in it? I would be interested to see if the tie is sitting in the top rotor. Possibly, the pin is loosening due to building vibration and has broken the tie?
The above said, yearly inspections and maintenance should catch all of this. I don't know what the protocols are at the Udvar. Especially if they were closed for a couple of years.
I forwarded this conversation to a friend that may have rigged this Sea King, or at least he may know who did it to see if he has thoughts on it. I'll let you know. Or he will.
T