I forgot to mention to check the insulators around the mounting screws when I posted last night. Hmm, I'm not convinced that the capacitance between the rectifier and the chassis because of the mica washer would take long enough to charge, that the meter would be able to register it. The orange ceramic caps across the diodes might be enough though and could cause that. It's also possible, that the fault doesn't show until some volts are present. Get that a lot with car ignition systems. You should really not be doing this sort of test with external connections to the diodes, the external components are likely to mislead you.
To rule it out it might be worth temporarily replacing those rectifiers with a bridge rectifier like this (ebay item no. 285437590123). It shouldn't break the bank and will prove whether or not the fault lies there.
As for keeping old things running, I share your sentiment, It just feels wrong to throw away something for a trivial reason. It's why I'm trying to create a sub-woofer using an EMI speaker the came in to my possession 40+ years ago, there's nothing wrong with it and I'm fairly convinced it will do the job, in spite of being pressured to throw it away and use a B139 instead. The people who think that are missing the point and are simply focused on the immediate cost rather than the value and the environmental impact of creating yet more landfill.
Same goes for classic cars, I've got one that probably is and another that may be getting there. I doubt I will be parting with them anytime soon.