I have one of these. They are a commercial product that clips on to a special boat hook and is used to pick up a mooring body as described above.
I inherited it with a boat and have never used it.
It's a BOSCO 1000 boat hook, it states it on the side. Offered back in the late 80's early 90's to solve the issue of hooking onto visitor mooring style buoys with the steel loops on top, or other hoop / closed bar type mooring bits, as others have said above. If I remember correctly the clips for the boat hook pole could also be attached to other boat hooks poles.
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The BOSCO wire loop for hooking poles Bosco Boat Hook, boathook, docking, mooring, buoy hook, Marine, Boat, Yacht, Tools, Tool
I have a 'hook' and no slider until I made one out of some electrical trunking that riveted on to a boathook. It's ok for picking up a mooring but you then have to swap out, etc for proper mooring lines.Handy until you lose the pole I guess. I have a somewhat large hook without a pole but never been able to find a slider to fit the bar on back of hook . It would seem to have a US patent

Is it the hook part of one of these? The Ultimate Boat Hook | Extremely easy to use | Hook & Moor AB
We have one and it’s excellent.
I had a similar hook to the one on the left. Never used it for buoys but it was big enough to grab the horrible scaffold type mooring arrangements on the ends of French pontoons and save the crew trying to thread a load of line through to secure things on berthing.
I have a 'hook' and no slider until I made one out of some electrical trunking that riveted on to a boathook. It's ok for picking up a mooring but you then have to swap out, etc for proper mooring lines.
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