That is called isolated field. We often see members upgrade to the higher output isolated field alternator then simply chassis ground the other field terminal rather than complete the upgrade. Makes no sense to me.
I upgraded my 67 to isolated field system with used parts and about 8 feet of new blue wire to complete the upgrade. Never new there was a issue with that solid state regulator. No indication shown in OEM amp gauge needle. When I changed it to a volts gauge that needle vibrated like crazy. New regulator cured it.