Hi 'Nando
,
I found a reference to a medieval plumb weight which was made from lead in the shape of a fish, with a hole for the cord, in the middle of the fish's tail. You could argue that the 'projectile' has a 'fishy' shape. I'm still thinking it could be a plumb weight used for larger buildings especially if the seller's date range is accurate
Perhaps a ballista / harpoon head ....the hole could be for a rivet to attach to a shaft or in the case of a harpoon head a retrieval line.
It does seem very well made for an item that is 'fired' once, but with a harpoon you would expect to be able to retrieve it .....but the lack of bad corrosion seems to suggest it didn't see a lot of salt water
All the best
David
PS Fearn,
I agree the 'whale hoist' has interesting similarities but does not have 'guide rings' to direct the dart via another rope. I think the problem with using this to retrieve items from the seabed are that first of all you need to be able to see it, water currents could easily push a 2lbs weight 'off course' and the item would have to be 'spearable'
Looking again at the 'dart'....the 'ring' which gives most of the weight has four flattened faces....as this would require more time/effort to produce, it suggests that these facets aid the function ...perhaps to prevent 'rolling'