From verso of card: "One sound locator is an essential part of the equipment of an Anti-Aircraft Searchlight Detachment and is used for directing the searchlight beam on to a target which can only be heard. It is manned by a crew of 3. Two of these are listening numbers. . . . The third . . . uses the sight which makes an allowance for the speed of the target; this member telephones instructions to the searchlight controller, who then moves the searchlight beam so that the target is illuminated."