Well, technically it's not. It was designed for the P1900 since people needed the 3rd output when powering their +12V screen from the secondary output. However, it DOES fit inside the P1280 enclosure. But you'll have to clip off one of the jumper connectors and solder it to the +12V pin on J1. It was designed to plug into a jumper header on the P1900 to derive its +12V input. This is no big deal if you are comfortable soldering.
I saw a post about the P5V, but didn't realise it was for the 1280 too.