The evolution of “adjustable” (i.e. single-face) traffic signals in the early 20th Century can be very roughly classified into three basic design patterns. Many, though not all, of the major signal manufacturers’ adjustable signals evolved through the following design patterns.
Solid Cast Signals:
Separate casts were fashioned for 1-, 2- and 3-lamp configurations. Engineers desiring, say a 4-lamp signal configuration would need to pair together two 2-lamp housings or a 3-lamp and 1-lamp housing.
Beginning pretty early, perhaps before 1930, tie-rodded designs were adopted by some manufacturers. To make signals easier to configure and repair, each lamp module is individually cast with open ends, and affixed by an access door on the front. The modules can be stacked together in any number, as required. The stack is enclosed by end plates at top and bottom and held together by two or three tie rods through the length of the signal head. The signal probably most well-known by collectors is the Crouse Hinds Type D/DT “Art Deco” which used this design from the ’30s well into the ’50s.
Taking the best aspects of earlier designs, solid cast individual lamp sections with a hinged access door on the front are simply bolted together as needed to create a traffic signal face. General Electric may have pioneered this design with their sectional signal introduced in 1937. The end plates and tie rods are gone, although they were still supplied extraneously for a few years with Marbelite signals when antiquated local specs required them.