Hi Simon, thank you for your drawing, very informative and well done. Generally speaking the wires should be labeled with coloured shrink wrap. The AC wires (one to the slip rings and the ones to neutral in the top box) are usually black, The +ve wire to the field is red and the -ve wire is white. Sadly many years of dirt heat and deterioration often make them indistinguishable. However, a close look at your photos shows two of the wires attached to the rectifiers appear to be brown, I suspect these are the +ve field wires.
I am hoping that one or other of the old rectifiers still works, if you disconnect the brown shrink wrapped wires you should then be able to test the two rectifiers with a multi meter. If you set the meter to Ohms, put the red probe on either of the AC terminals (connected to chassis or slip ring) then place the black probe on the vacated rectifier outlet you should get a reading. Now reverse the polarity of the probes, you should get no reading. If this is the case then the two wires with brown shrink wrap are indeed the positives for the auxiliary field windings.
Let me know what you find, please be careful with old selenium rectifiers their contents are toxic.
Bob