That's the wire to your anti-diesel solenoid (AKA Idle Stop Solenoid). It controls the throttle opening at engine idle to prevent dieseling (Run on) after the ignition is turned off.
The solenoid is energized when the ignition switch is turned on,
causing the plunger to open the throttle to idle speed position. The
plunger length is adjustable so that the idle speed can be adjusted. When
the ignition switch is turned off, the solenoid is deenergized and the
throttle closes tightly, cutting off the airfuel mixture.
Factory Part number for your 1971 is: D0AZ-9D856-A