This thing is for mounting shelf boards of arbitray length to your IKEA Ivar shelf.
There are two versions. With BoardSupportNormal.stl the board will be at the same level as the original shelf boards. BoardSupportOffset.stl raises the board halfways to the next level.
For each board you will need four supports, two of them printed as is and the other two mirrored around the x-axis.
DrillTemplate.stl if for marking the location of the 5mm fixing holes that need to be drilled into the lower side of the board.
You will also need the appropriate metal pins. They can be found on ebay, or you buy a 6mm round bar at your local hardware store and cut 40mm pieces from it.