Hard to tell from those pictures but I'll take a stab at it. I'm assuming an "islet" is a small island.
If that's the case, I'd think it's off a vessel. If it's soft, probably lead. Holes...for mounting.
I'd guess an anode off a boat. Possibly ran aground and busted it off or old deserted boat or boat part washed up and that piece rotted off.
An anode is placed on a boat to help prevent corrosion.
