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It is a Pelorus, sometimes called a "dumb compass". It is used to establish relative bearings to other vessels, islands, land marks with reference to the ship or boat's heading. Small ones like this are primarily used for inshore navigation. In use the 0/180 degree line is set parallel to the vessel's keel and clamped in place. Sighting is done by lineing up the slots and wires in the sight vanes (which are missing on this item) with the target object.
Navy ships have larger more elaborate examples of these on the Port and Starboard bridge wings to assist in navigation, position keeping when sailing in formation and fire control.