Join Date: Aug 2010
Location: Fort Frances, ON, CA
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I'm guessing short bolt inserted from below, and a threaded coupler. There are "chew marks" on the coupler that are consistent with using some sort of pliers to tighten them. This would certainly keep the bolt from sliding out and getting lost if you accidentally removed the coupler.
Disclaimer: I'm not a rigger, and I'm only judging based on your photos. Perhaps someone else can confirm.