[Arran]

I would have to look at the schematic but it's not uncommon in 1930s AC radios to have the negative of one filter cap connected to the center tap of the high voltage winding, and the other to chassis, with a series of wire wound resistors in-between. It's a common mistake a lot of people make because they are used to working on 1940s and 50s radios that have the tubes cathode biased rather then fixed bias, so in effect one filter cap is open circuit if you connect both to the chassis.
Regards
Arran