The “2nd Detector” 227A triode is being used as a diode, with the plate shorted to the grid. The potentiometer in question would adjust the DC plate bias current. I don’t have any data on this tube type, but I suspect it would be nonlinear, and the pot would adjust it to the most nonlinear spot, maximizing the detected audio signal. To adjust it properly, I would use an amplitude modulated signal from a generator, and adjust the pot for the maximum audio output.
