DID A GOOGLE ON
BC-608-A Signal Corps Found This:
http://www.qsl.net/vk2dym/radio/iff.htm"CONTACTOR UNIT BC-608-A. The clock was mechanical and hand wound"The clock was mechanical and hand wound, preferably every 12 hours, although it could run for 24 hours. In operation, the pilot would actuate the clock STOP-RUN switch before take off. Thereafter the aircraft transmitter would be actuated to send out an unmodulated signal for about 14 seconds of every minute, ie. in the 12 o'clock to 3 o'clock quadrant indicated. The transmitter switching could be disabled by the contactor IN-OUT switch, leaving the clock to continue running.
When the contactor was in operation it meant that the pilot could not use his transmitter or receiver during that 14 seconds, or if he was in the middle of transmission when the contactor came to it's 12 o'clock position, he would be cut off.
This system was fitted mostly to fighter aircraft and interconnected to the common SCR-274 (Command) sets in US aircraft and the TR9 in UK fighters. Later the Contactor was fitted to the VHF SCR-522 sets as they were introduced in aircraft. Here position D on the 4 position push button channel switch was used for DF. A relay in the SCR-522 transmitter caused a 1000 cps tone in the headphones so the pilot knew when pipsqueak was operating.
In practice, it is doubtful whether it fulfilled it's intended purpose of identifying the location of friendly aircraft and it was probably a pain for the pilots, having to watch that clock before transmitting a message.
