[Ron Ramirez]

The voltages were originally measured at the factory with meters with a sensitivity of 1000 ohms per volt. That's all they had at the time. Those meters loaded the circuit under test, resulting in voltage readings that were actually incorrect!

Today, we can easily buy digital multimeters with input impedances in the megohms per volt, which do not load down the circuit under test as did the old meters from ye olden days.

You can acquire an old early 1930s voltmeter...or find a low-end analog multimeter made by Calrad and sold in the 1970s...or maybe someone will measure the voltages in their working model 20 and share the results here. I no longer own any Model 20 sets.