This is the V-Slot Spacer Block.  It was specially designed to be used in conjunction with the V-Slot Gantry Plate.  
It allows you to raise the V-Slot Gantry Plate up away from the rail 12mm so that you can run transmission components such as a belt drive or threaded rod underneath the V-Slot Gantry Plate. 
M5 tapped holes to allow mounting to the V-Slot Gantry Plate.  
3 holes that allow for mounting of full size OpenBuilds Wheels.
SolidWorks parts are here