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Alan, if you have "knobs" that are labelled for lighting, they areprobably dimmers (potentiometers) that allow you to dim the night lighting to the lowest level in order to preserve your night vision.
Pots are subject to wear and corrosion so they may not be working any more. They usually can be replaced pretty easily but most folks would just use colored lighting (red isn't the only choice) with a plain switch. I've never seen dimmers like that on a boat, would suspect they were an "owner special".