[Arran]

No Kirk;
  In this case you are not the dumb one, whomever designed this thing was the dumb one, or crazy. At first glance I thought that maybe they had used the 25Z5 for a voltage doubler, but no, one diode does absolutely nothing but power the field coil for the speaker. In most of these early AC/DC sets they connected both of the diodes in the 25Z5/25Z6s together in parallel.
 Now you would think, hmm, AC/DC set, department store set, so cheap set, but these clowns added an extra filter choke doing what could be done with the field coil on the speaker. I can only speculate as to why they did this? A trainload of speakers for DC only sets that had no hum bucker? A rail car full of filter chokes they had to use up? Patent royalty circumvention?
 In any event filter cap C20A and C20B and paper cap C19, one end of the speaker field coil, and one end of that filter choke are all connected to the same place electrically, to the switched side of the power line which is B-. Capacitor C20C is connected to the other end of that filter choke, which is connected to the common negative for the rest of the set.
Regards
Arran