The development of photographic plates (smaller size and greater sensitivity) encouraged manufacturers to develop a less bulky equipment, cheaper and lighter: the "detective". This name comes from its cubic shape and its black color makes it more "discreet"!
The plates are inserted in series of 12 or 6 with a process retracting lever which ensures that each impressed plate falls into the bottom of the box and the other is in front of the lens. The plates were often 9 x 12 cm size or 3 1/4 X 4 1/4 "called "quarter plate."
Later, some detectives will have a special exchangeable back magazine for roll of film or plates.