Hi
Nice puzzle for passing the time while locked in ATM.
It looks handmade and if so it should be for a specific job. It looks like it would hold 3 seperate lines that could be individually slipped off by pulling each hook back. The shackle is quite small and so would only be for a few pounds total and so each line a third of that.
My first guess would be to hold
fishing lines out of the way while sailing, second guess is still forming.
I hope the original owner comes on with the answer