No, it's not hollowed out.
Hnikar, is 5 cms not too short for such an item? I am interested in your idea, because I had thought of that, too, but you have an ethnological reason, too. The ones used today, are they that small? I guess it'd fit nicely in the hand if it were smaller.
Two things:
1. I'm told the bone is very soft, and yet there is no wear on the piece.
2. Don't get too stuck with it being a harbour. It was a reasonably busy area in itself, with all sorts of comings and goings.
I had in mind one of those toys where you have two sticks with a string in between. Then you have a little weight thingy that you put in the middle, and if you work the toy just right, with skill, you can roll the weight back and forth between the sticks. But then you'd not have such a wide "waist" on the weight as you have here... It could just as well have been a sewing implement as a peg for a game of some kind, or part of a tool for navigation.
Thing is, that the harbour itself would be considered "viking", but seing as how it is a harbour, whoever visited and dropped this, could just as well have been Ibn Fahadlan, or some dude from Italy, so don't get stuck on the Norse thing either. All input is appreciated!