Well, because the thing looks custom made. I dont think its the same box that fits into the empty VCO slot.
The area where the VCO is missing from has three screw marks, and the external "vco' module measures 52x54mm.
If the external VCO module was placed internally then it could at most use one screw, due to the spacing of the holes and the dimensions of the box, leaving two screw marks unexplained.