[Arran]

 First something isn't correct here, the set can't have a 25Z5 and a 36L6 tube, one has a 300 ma heater and the other a 150 ma heater, and this is a series string set. So either it uses a 25L6, 25A5, or a 43 for an output tube.
  Unfortunately I can't read the schematic thanks to this P.O.C computer, but in any event four wires coming out of that condenser box could mean a couple of things. One could be that the set uses a fixed bias supply, like one in a pre war AC set, where one filter cap has it's negative connected to the most negative point in the power supply and the second one has it's negative end connected to a slightly less negative point usually to a resistor running between that point and a more negative one. Another possiblity could be that the included some paper caps inside that box, like large bypass caps.
Regards
Arran