This is a New Zealand favourite. The size in mm relates to the diameter of the galvanised bar they are constructed of
A lightweight four pronged grapnel constructed of steel and then galvanised.
An excellent rock anchor for fishing or in many cases on the end of nets and trawling for that main anchor you just dropped overboard! If the grapnel gets locked in rocks then one fluke will pull straight with heavy force and allow you to recover it from the rocks.
A real fisherman's friend.
Another best use is to throw at a mooring buoy or net buoy for retrieving. Also if you lose a fishing rod overboard drag one of these along the bottom, they catch onto most things.