We may be getting somewhere.
Each ATX has the green wire connected to the black ground inside the box.
he remaining black wires go the the red of the next box.
The mount is plywood and there is a piece of plywood between each ATX.
Here's another clue.
If I use a 9V battery the meter reads right.
If I use a AC/DC 9 V plug the meter is not correct.
I believe that the AC supply is having its effect.
I am using only 2 wired for the AC.
I don't know whether the ATX cases are at ground potential.
I was careful with the polarities while soldering the 2 wire connections.
The output is certainly ~ 15 VDC.
The 10 Volt point measures 10 Volts.
Still confused.
BoyntonStu