This is a adapter for a Apple G5 case mod / Hackintosh mod where you want to have the power cable removable and not leave it in the PC case stuck on to the PSU and then the wire hanging down.
You take a PSU cable and cut the wall plug away, take a power plug from a old PSU and solder it to the cable. With that you get extension cable that is always in the case and no need for opening up the case and unplugging the cable from the PSU.
The opening in the case for the original power plug is a bit to big and to wide for the standard PSU plug so the adapter just snaps on to the power plug and then it covers up the opening. The holes in the adapter line up perfectly with the perforated back panel so 2x M3 3mm screws are used for fixing it into place.
And the job is done ;)
here you can find more picture of the mod