A "booby trap" in Portsmouth Dockyard, Hampshire

The subject of this negative (and negative number N26416) is given in the Portsmouth Dockyard Negative Register as "Booby trap". The precise location in the dockyard is unknown but the crane in the left background looks like one of the spider cranes located on the west side of No. 3 Basin between No. 9 Dry Dock and Lock A. The only common denominator of these two negatives is the small box next to the circular object on the platform on the right, at the top of a small flight of steps. Vertical format.