Ah, yes...I see you indicted that. No, this wire looks to be part of a mod when those bias and filter caps were replaced. Those resistors that you noted are part of the rectified negative bias voltage that wind up on pin 5 of the power tubes. It looks like some one rearranged the layout a bit and that orange wire could be anything. If I had to guess it is a ground jumper for one of those resistors...But you will have to verify it by tracing that bias circuit and seeing what reaches gound and doesn't. What year and which Mark II is this? A IIA or IIB?
Not to be critical but the smaller staging B+ caps(the blue 500v ones)are in rather sloppily. The crucial staging resistors that drop down the voltage are not where they are suppossed to be. What I see is some extra long lead length from the old caps. I am assuming the resistors are soldered to that. If you can take a pic of the whole chassis we can verify what is looking good and what ain't...