Squareback vs round back alternator?

Ha!
I just looked at this screen shot again and see that slide does not show voltage drop measurement.
voltage-drop-jpg-jpg.jpg
Rather it shows voltage at the ignition connection. Subtracting that measurement from battery voltage will reveal voltage drop.
As described in the booklet, when using analog meters a more sensitive reading of voltage differences can be made by measuring between the battery positive and the ignition connection.
Also its taken from the ignition circuit test. The 0.4 Volts is with the field disconnected and the ignition points closed.

The following illustration is from the Field Circuit test.
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In this test the engine isn't running and system voltage will be lower than the regulator's set point.
Therefore the regulator will allow maximum current through to the rotor.

When testing for voltage drop with the engine running, the test lead will have to be on the regulator's input terminal.
The other test lead should be used to measure the alternator's output voltage, and then also the voltage at the battery positive.
Be sure the battery has finished charging before measuring for voltage drop.

It looks to me like chrysler really wanted no more of 0.2 Volts lost when the alternator was running the engine and one accessory.
That's my interpretation of this 1960 test.
1960 Imperial and Chrysler Service Repair Book - Alternator Service
In it, the output is controlled by engine rpm. I'm not really thrilled with that idea.
I think it would be safer to leave the regulator connected and add a load to the system thats approximately 10 amps. For example, heater fan on medium speed, or lights on.
That's how I've been testing voltage drop. When I can I measure the amps with an inductive clamp.