Contactor Unit BC-608-A of RC-96A System Pipsqueak
Here is an excellent example of the WWII-era Contactor Unit BC-608-A, a component of the US Army Air Force's RC-96-A Contactor System intended to work with the the British RAF "Pipsqueak" system.
Pipsqueak was a simple radio navigation system used by the British during the early part of World War II. Pipsqueak used an aircraft's voice radio set to periodically send out a 1 kHz tone which was picked up by ground-based high-frequency direction finding (HFDF, "huff-duff") receivers. Using three HFDF measurements, observers could determine the location of friendly aircraft using triangulation. An excellent overview of the system can be read at the Duxford Radio Society website.
RC-96-A was designed for use in US Army Air Force aircraft to control a radio transmitter so that it will automatically transmit a predetermined type of signal on a predetermined carrier frequency for a period of approximately 14 seconds of each minute. The control circuit in the contactor unit consists of a cam-driven set of contacts. The system was used in early versions of US Army Air Corps/Air Force fighters such as the P-38, P-39, P-40, P-47, and P-51, typically with radio command sets SCR-274 and SCR-522.
According to the Smithsonian National Air and Space Museum website:
The “pipsqueak” identification unit contactors like this one were the solution before the advent of dedicated IFF transponders. They were essentially timers for communications radios that broadcast a fourteen second carrier wave every minute, which allowed the RDF stations to determine a fix (but also prevented the pilot transmitting during a period indicated by the arc on the dial). The BC-608 “Pipsqueak” was an American variant of a British unit developed in 1939 that entered production in January 1942. As the Chain Home system grew in capability and coverage, the need for the contactor disappeared and true IFF (Identification Friend or Foe) transponders took their place. Production was only about 500 units.
Once wound, the Contactor is ready to go. Flipping the toggle marked "Contactor" to "In" connects the aircraft radio to the contactor. Flipping the toggle marked "Clock" to "Run" starts the clock, which will continue to run until flipped to "Stop" and the clock hand reaches 12 o'clock position.
This example has been installed in an aircraft during its service life, as evidenced by the trace of zinc oxide paint near its screw mounts. The clock runs as it should, and all toggles operate as they should. Measures ~3.25 inches diameter and ~4 inches deep.