[Arran]

"Does it have anything to do with the connection to the 6F6 output tube grid and its bias?"

Yes it does, the most negative point in an AC radio is the center tap of the power transformer. So the way they developed the bias voltage was by connecting a large wire would resistor with taps, or series of resistors, to the center tap to make everything else more positive with respect to the center tap including the chassis, the chassis would usually be at the end of the resistor network.
This is a fixed bias supply and was very common in pre war AC radios. So to an electrolytic cap the chassis would be the + side and the center tap would be the - if one was installed there. Later on they used a paralleled resistor and capacitor between the cathode of the power output or other tubes and the chassis to make the cathode more positive with respect to the grid.
Regards
Arran