The cantilever 'Hammerhead' crane is only one of six built in the world. The crane was specifically set up for the removal of the twin 18 inch gun barbettes (barbette = an armoured cylinder protecting a revolving turret on a warship) from the English battleships of the period, hence their location at the home ports of the Royal Navy. (quoted from http://www.dhub.org/object/319851&img=174519)