My take: The GM 10SI alternator #1 terminal connects directly to one end of the alt. field winding, the other end of which is connected to the collector of an NPN transistor whose emitter is grounded. The transistor is biased "ON" by resistive path to #1 terminal. So, when + battery is applied through a lamp, resistor, or diode to #1 terminal, the Alt. field is energized at a low current. The #1 terminal is also directly connected to the diode trio + output, which substitutes its + voltage as full power to the regulator and field when the alternator starts to charge. Then the current through the lamp/resistor/diode falls to zero. Without a diode/resistor/lamp to limit current, then the ignition system will take its power directly from the diode trio + output and the engine will continue to run with the ignition switch "OFF". The diode blocks power feed from the diode trio + to the ignition, but not battery + feed to the Alt. regulator before charging starts. A resistor or lamp has to have enough resistance to prevent coil ignition and 'run-on.'
The #2 terminal supplies battery + through a very low current (high resistance) voltage divider/zener diode biasing circuit to the base of a second transistor whose collector-emitter circuit shunts base-biasing 'on' current away from the first transistor, thus lowering field current when the battery voltage rises above an amount set by the zener diode and voltage divider.
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