Looks like an emergency tiller to me. Had to use one once, always made sure I knew what it looked like and where it was on any boat i've sailed ever since
One sure fire way to find out what it's for is to throw it away.
Very soon after, you will realise that it was essential for some task.
Been there, done it.
We need some dimensions to have a more positive identification but if that rod is about 10 mm diameter with a square machined on the end I suspect it may be a replacement handle for a roller reefing gooseneck fitting.
On the otherhand if its about 25mm diameter or bigger i would suspect an emergency steering tiller.
PS just looked at your profile.Your boats not likely to have old fashioned roller reefing so I guess it could be a key for removing a plug from something .
Well its identical to the one I used in the French canals...... i'm led to believe that there are many different types from sockets, to splines, both male and female... depending upon where you use them....
Barton Reef handle are some people trying to say ?
Nope - as it hasn't got the ridge at end where "hand-bob" is fitted so that you can hold it and turn without the metal gouging your palm to bits ..... unless someone turned it up themselves ... without handle ?
I have a packet of metal tangs lieing in bottom of a locker .... couldn't fathom out those till it dawned on me - they are locking tangs for bottlescrews !! the ones that have the small tab in the body to lock it from turning !