The red test button confirmed if the ADF needle was live, i.e. not stuck.. The black knob to the right was for volume (audio output).
One would have to consult the Bendix Operator's Manual, but CW could also mean Continuous Wave: i.e. Morse Code transmission.
A continuous wave or continuous waveform (CW) is an electromagnetic wave of constant amplitude and frequency; a sine wave. In mathematical analysis, it is considered to be of infinite duration. Continuous wave is also the name given to an early method of radio transmission, in which a sinusoidal carrier wave is switched on and off. Information is carried in the varying duration of the on and off periods of the signal, for example by Morse code in early radio. In early wireless telegraphy radio transmission, CW waves were also known as "undamped waves", to distinguish this method from damped wave transmission as effected by early spark gap style transmitters.