Re: is this a boom vang?
That's a boom vang and attachment for a mainsail that is reefed by rolling it around the boom. The semicircle yoke 'thingy' accomodates the rolled sail, but also is tight enough to grab onto the boom when unreefed.
It would need to be slid onto/over the boom with the gap up before connecting the mainsheet.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)