On the Oilheads, the alternator is bolted to the vertical support bracket with spacer blocks in between. On top there is a simple block, on the right there is a strap connecting the alternator to the bracket, and on the left there is the adjuster. The top and left bolts enter the alternator housing from the back, then go through the spacers and through the support bracket. On the front, nuts lock the assembly in place after the belt has been tensioned (which happens by the alternator being moved up and down in slots).
However, it's not quite that simple. The top bolt actually threads into the spacer block, holding it tight to the alternator housing, and this whole assembly slides on the support bracket. What happened on Meredith's bike was that the upper bolt had backed out during a belt change without my noticing, and while the nut was tight the alternator itself was loose. In other words, I had clamped the spacer block tight to the support bracket but the alternator was loose on the head of the bolt. I found this because when I tensioned the belt and secured it with the upper nut, the alternator still moved when I released the tensioner.
The fix is obvious once the problem is identified. Loosen the nuts, tighten the top bolt and THEN tighten the nuts again. Problem gone.