Voltage Regulator problem

I would start at the alternator and follow the 1 field wire back to where it comes from. I am not aware of a 1wire (internal regulator) alt conversion for square back alternators.

As 67dart273 pointed out, not sure how it ever worked as is right now.

Quick test. Disconnect the field wire in photo 2. Measure resistance to ground on both the field terminals and report back