Power supply with three pins on the output, why?
I found a thread on BadCaps.net Forums discussing a different printer with this specific power supply. The schematic was user-generated so YMMV.

Pin 1: +24V
Pin 2: On/Off Control (marked as ON/OFF 3V)
Pin 3: GND
The on/off pin is coupled to an N-channel MOSFET which gets tied into the main feedback 431/optoisolator circuit. If the MOSFET is off, the voltage looks like it would be set to around 8.5V. If on it adds parallel resistance to the divider and the output would be around 24.3V, so this makes sense.
It most definitely is not a dual-rail output. And there is only one transformer (the coil near the AC connector is a common-mode EMI inductor).