Connectors:

 sw - yl: control speaker

 yl  - rd: 400 Ohms output winding

 triple coloured: feedback windings

 c - red/red - c: plate #1 - B+ – plate #2

  Originally the output impedance is 400 Ohms with four secondary windings in series.

  Wired in parallel the output impedance will be 25 Ohms (Maintaining the same primary impedance). Still OK for 16 Ohms speakers.

Original secondary winding: 

 sw - yl (twin): 1.3 Ohms

 yl (twin)  - gr (right): 5 Ohms

 gr (left) - br (left): 7.4 Ohms

 br (right)  - yl (right): 7.4 Ohms

 yl (left) - rd: 6.3 Ohms

Conversion to 25 Ohms secondary:

1. Solder the two windings in the yellow isolation together and isolate them

2. Parallel the four windings you get taking care of phasing. All reds from the picture and all blues have to be soldered together.