charging system blues

The other alternator is wired for the earlier style voltage regulator.
On your '71 voltage regulator:
Blue wire monitors system voltage. The regulator decides what to do based on this voltage.
Green wire is the alternator's grounding wire. The Voltage Regulator controls the power flow through the field to ground.

Voltage regulators before 1970 control the power to the field positive. Field negative goes to casting which is used as a ground.