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I rebuilt a very old GE BT-1-A FM mono transmitter. While doing some extended “testing” of the transmitter I simply added a 19 kHz tone from a signal generator to the program audio. At just the right level the stereo indicator lamp on receivers would light up.
Maybe those of you that have mono transmitters can give listeners what they expect to see by adding the tone and lighting their stereo lamp. Most people don’t really know what stereo is anyway.