looks like one of those gadgets for splicing multiplait or braid.
For picking up a mooring ring?
It's for picking up a stern mooring buoy, they are common here in Finland.
More on Scandinavian mooring here: http://www.ybw.com/forums/showthread.php?11234-Scandinavian-moorings
Edited to add, it actually hooks on to the buoy, and is left in place there, it's not for threading a rope.
So that's a spring loaded plunger running down the middle of a tube is it?