Maybe "PO" is "Panel Out" connected to the front panel + terminal (either
AT or GND would be the minus terminal then).
This one is not the same as one I have but the PO terminal on mine is used
for current shareing when many units are opperated in parellel 'Parellel
Opperation' If you trace it back it should go to the input of an op-amp.

Cheers Steve.....