I've been studying the power supply as well. Here's the pin-out as far as I've derived:
I believe the Power ON to the 6-pin connector is an input, and the one on the 16-pin supply is an output, as the former is the input to a photocoupler, and the latter appears to be connected to the collector of a transistor (?) Q102.
The +5V is always on as far as I can see, and is used to turn on the power supply through the photocoupler-connected lead. The power supply might be be providing 12V as well, or just staying on because the power on is floating. I'll have to test it later when I splice the cables.
Edit:
Power ON on the 6-pin connector is active-low. You're measuring 4.53V because of the diode in the photocoupler and the resistor in-line. The anode (+) is connected directly to the +5V input on the 6-pin connector.