It's a voltage doubler. I use that a lot because it's full-wave and I can use 600 volt Schottky diodes, which are MUCH cheaper than the 1200 volt versions. (These high voltage ones are silicon carbide, made as far as I know only by Cree.) A side benefit is that the winding is about 160vAC, and switches rated for up to 240vAC are readily available if anyone wants to switch the high voltage later than the heaters.
The second filter stage is a pair of RC stages, one for each channel - what we used to call "pseudo-dual-mono". There is a large resistance for the 45, with a smaller, optional parallel resistor to raise the voltage for the 2A3. This is intended to make it easier to change from one to the other.
The 2.5v windings are separate so that independent cathode bias can be used. It's actually a quadrifilar winding, with a pair of wires for each 2A3.
The windings are arranged so they can provide some degree of electrostatic shielding for the primary and for the 2.5v windings. You can probably tell that I am a little proud of some design aspects... :^)
