I have finally managed a couple of (more) treasure hunts. Two beaches over two+ hours, over two days.
And I have this Whatsit.
The head is Lead heavy, the spring still works, only the spring attracts a magnet.
The old 'cap bombs' come to mind, but I don't know. It has to be contemporary.
Mike
a paper cap was torn off a roll of them,placed under the spring end,tossed in the air where the heavier bullet nosed end struck the ground causing the cap to go off