The "Command" radio is a description of the radio function, rather than a specific model. The "Command" function was to communicate between aircraft. The "Liaison" function was to communicate back to base. However, functions often become blurred in use, and even though there were several different models and generations of "Command" radios, one particular model has become popularly known as "Command sets".
The Command set is a general title for several groups of receivers and transmitters used in World War 2, which are more specifically called the (United States Army Air Force) SCR-274N, (United States Navy) ATA/ARA, and the (United States Navy) AN/ARC-5. They were used for Air to Air, Air to Ground communications and for receiving Navigation signals. They are a unique design of that era, in that most radios would use a band switch to change to a different frequency band, whereas this design selects a completely different receiver or transmitter. After the war, they were still used.
The Army SCR-274N series radios were available in black crackle paint, or natural aluminum finish. The Navy ARC-5 were in black crackle paint. Externally they looked identical in appearance, and only differed in color and frequency coverage. Internally they were very similar, with only minor differences, apart from the obvious ones relating to the frequency. The SCR-274N series had BC-454 type names and the ARC-5 had R25 and T20 type names for the receivers and the transmitters. The adapter drawer on the front had a different knob and label for Army and Navy. Some of the low frequency receivers had a different antenna connection so that a loop aerial could be connected. The circuits were almost the same, with only a few small changes because of the frequency coverage. The valve line up was the same, except that some ARC-5 receivers used a 12SF7 second IF valve instead of the 12SK7 which was more common.
They are a very nicely made radio. The metal parts are aluminum pressings, with rivets and riveted nuts. The capacitors, chokes and transformers are in cylindrical metal cans with mica insulators. The resistors are mounted on mica tag strips. The wire is a cotton covered type, sometimes with a tropical proofing. All the connectors use mica sheet. The RF coils, IF transformers, and dynamotor all plug in.
They can be operated locally, using a tuning knob and an adapter. Normally, they would be remotely controlled using a control box. The control box had electrical connections for the Gain and MCW/OFF/CW switch, and a flexible tuning shaft for the frequency.