photo caption - Electronic Time Division Multiplex Unit with attached battery of four Teletype transmitters. The Electronic Time Division Multiplex rearranges the output of two to four teletype machines so that the signals originated by those machines are transmitted over a single radio channel by a single radio transmitter, simultaneously for all practical purposes. The apparatus is divided into transmitting and receiving units. As messages are fed into a battery of four teletype transmitters hooked into the ETDM each is compressed in time, sorted into the proper time channel, mixed with the other signals, and fed into the radio transmitter. At the receiving end, the process is reversed and the signals, unscrambled, are fed into the teletype printers essentially at the same time as they are transmitted. Since each teletype transmitter can send  at speeds of 75 words per minute, this means a total of 300 words per minute transmitted. The Electronic Time Division Multiplex is calculated to make the best possible use of available radio frequency channels in military operations.

The sign in the photo says that it is being tested on a 6000 mile RTTY loop DC-SF-DC.

More info on this AN/FGC-5 system is at