Here is a question to answer first: does the switch work now?
It is a plain old DPDT switch, center off, right? Looking at the photo, the top row of solder lugs is one switch SPDT, and the lower row is also a SPDT.
The switch action is simple. Center is off - amp is off. FLip the switch one way and the amp comes on warming up, but no high voltage to the circuits. We call that standby. Now flip the switch past the center off over to the ON position, and the high voltage is fed to the circuits. The only difference between ON and STANDBY is where the high voltage goes, or doesn't.
IN other words, the main AC is on either way, and the B+ is on only one way.
Looking at the photo, I see the mains white wire going to the center of hte top row. The the two ends of that row are wired together with the short orange wire, and the orange from the PT connected to them as well. Assuming the orange wire is one of the power tranny primary wires, this means that the orange wire will be connected to t he white mains wire with the switch turned either way. The black mains wire goes to the fuse, and a white wire from the other end of the fuse completes the circuit to the power tranny primary. SO far it seems we have a basic on off on AC mains switching circuit.
Now the lower half of the switch. A brown wire from the PT goes to one end of the switch, and a red wire from the center lug runs out of sight, presumably to the HV rectifier. With the switch centerd, everything is off, with the switch thrown one way, this lower half completes the circuit to the HV rectifier, flipped the other direction, nothing is connected so it remains without HV.
So unless I am not looking at it right, it ought to work as advertised. Flip the toggle towards teh end of hte chassis, and the mains comes on to warm up the tubes, then after a moment, switch it to the other end, and the B+ comes on too. Does it not do this?
And if it does do this, why rewire anything? Looks to me like someone had an amp with NO standby switch and just had to have one, so they did this so they had a standby without drilling a hole for a second switch.
From the center off position, flip the switch one way to warm up the amp, then a moment later switch it to the other way and the amp can be played.