This is a mounting bracket to attach a marker to a Printrbot to create a make-shift pen-plotter.
It may need some filing on the lower lip of the hole - the marker should be inserted up from below, to a depth where the marker tip is a bit below the extruder tip, and secured with an M3 size screw - the plate is thin enough that it is possible to mount it (at least on my printer) between the Extruder Mount and the Extruder itself.
The STL the SKP file produces is imperfect at the moment, so the uploaded STL file has been passed through NetFabb for cleanup.
This is a derivative of the pb-Xtruder-Mount-v2.skp posted by Brook Drumm.
P.S. I forgot to mention the first time, it appears the distance from the holes to the increase in thickness was about half a millimetre too short on mine, so I also had to file those edges down a bit.
The version 3 files move the height increase back 1 mm, and use a straight-through 7 mm hole instead of a tapered hole for the pen.