Ign power to reg and alternator. very similar to the welded splice.
Squareback mid 70’s alternator. tried the wires directly on battery circuit with same result.
i will recheck ground, but i doubt its a problem.
Where are you measureing voltage?
other possibility is the regulator is not breaking the ground - assuming its negative controlling VR.
Is it the brush or wire grounding?
If not then maybe bad regulator.