Vector monitor, purpose of a third "green gun" connection
I think what you're looking at is a tool for calibrating chrominance (color burst) signals in old analog TV signals. That would explain the labels of on the axis. The vertical axis measures the (R-Y) the red difference with Y, luminosity and the horizontal axis measures the (B-Y) the blue difference. Thus, the red and gun switches seem to ground entirely one of these axisis so you view just a single component of the your 2D color space.
I am guessing that the input is RGB and the output is the color space, thus jsotola is correct in that the &quot;green gun&quot; is in fact just the input for the green input signal. I'm guessing that the green signal ouput on the vector display is actually just the magnitude of the signal.