There are two material versions known: The early versions were made from aluminium, they have to be unscrewed very carefully. Copper grease was not so common in yesterdays garages, so the aluminium nut could easily corrode on the light alloy cylinder head.
Later they used a nut made from a copper zinc alloy, which was chrome plated. This copper zinc alloy was more robust, there were no damages to its crown when backyard mechanics tried to disasemble the exaust system. A special tool with the shape of the nut’s crown the Victoria officials did not regard as necessary.