[Arran]

I had a look at the schematic I linked to, I'm not sure why someone would play with the screen grid voltage on the RF amplifier tubes as there is no hint of this being in the original diagram, it's pretty much a case of where they alter the cathode voltage on those tubes to make the cathode less or more positive or negative with respect to the grids, and the section for the antenna circuit basically ground out the antenna circuit via the wiper. I could see eliminating one, and keeping the other, fixed a Westinghouse 801 that altered the cathode voltage alone I think, but they left the screen grids alone. If you can't find a dual control like this then I would go with either the 50,000 Ohm one on the RF tube cathodes, or the 30,000 Ohm one in the antenna circuit.
Regards
Arran