Thanks for the replies .....as I thought, not likely catapult ammo. I agree that it probably was 'hung', I think it may be a 'heavy duty' plumb line ....the spear end providing an accurate pointer.
Fearn's is a good suggestion, but I think using this as a 'sounding' device, runs the risk of it being snagged/embedded on the bottom, or giving false readings if the 'bottom' was soft silt and the item embedded itself deeply.
Thanks Mark for the links.The whaling connection sounds interesting....and if it is right, the buyer probably did well....Whaling implements seem to command high prices.
Still, it could even be something else
Kind Regards David