squareback alternator wiring in pre 70 car issue
... I also did a test from the + on battery to the blue wire/ign on my VR and get 12.20V without the alternator field grounded which is the same I get by testing the battery alone at idle.
A gazillion posts about this, but most people need it summarized for them.
By unplugging the field gnd connection, you disabled charging and thus see just battery voltage (should be 12.6 V fully charged).
When charging system works correctly, you should measure IGN to BATT- as 14.3 V. I bet your VR is good and you would measure that,
right at the VR.
The VR can only control what is sees. You have minimal drop thru the gnd connection, so that is good. Personally, I don't trust sheet-metal screws so run a dedicated gnd wire to the case. You likely have excessive voltage drop from BATT+ to "IGN
at the VR". That path is thru the bulkhead, key switch, bulkhead again, then VR. The bulkhead is ususally the problem. Search "MAD Bypass". That disables the dash ammeter, but you can use a cigarette-lighter voltmeter to monitor charging while driving. Smart on long trips so no surprises. Harbor Freight sells a cheap LED-light version, but lay out $15 for a better digital one (Amazon, ...).