All good questions! Unfortunately without a schematic, I can't really tell you. I haven't had a chance to trace the circuit yet, so you know as much as I do at the moment.
I've also noticed that someone's bunged two diodes in there, which I suspect isn't original either, so it may be that they're trying to bypass the rectifier valve. They look to be in a strange place, however, hanging off the voltage range pots, so I'm not so sure.