You MAY have an open in the charge wire AKA into the bulkhhead on the big black, through the ammeter circuit, back out the bulkhead on the big red, through the fuse link and to the battery
As Dana said, the IGN switch feeds +12 to one field terminal (does not matter which) and the VR effetively "controls the amount of ground" on the remaining field, (I think it is pulsed) which controls the field current and the output of the alternator
So to check the field circuit.....Unplug the VR and jumper the green wire connection to ground. Start it up, slowly bring up RPM and check voltage at the battery. IF IT does not come up, move your meter to the output stud on the alternator.
If this does not work, then it may be harness wiring. Go to the alternator, disconnect the green field wire and jumper that to ground, with the key in the "run" position. In reduced light you can usually see AND HEAR a small spark. With this connected, take your meter and confirm you have nearly battery voltage at the blue field terminal. DO NOT disconnect the wire, check this by "backprobing" the terminal. If you cannot reach them, switch the connections around, AKA move the blue to the opposite field terminal and ground the remaining one.
If this checks out, try again, run the thing as it sits connected, and recheck battery and alternator output stud. If this works, check the green wire for continuity.
The VR MUST BE GROUNDED. "Work" the VR connector in/out several times to "scrub" the terminals, and feel for tightness, and inspect the female and male terminals closely with a pocket light.
This is a simple circuit. The key feeds "ignition run" to the blue VR terminal, the ignition system, and the blue field wire. The VR controls "grounding" on the remaining field
The black output wire goes through the bulkhead, through the ammeter, back out on the "big red" and to the battery.