that is a jig for drilling the ends of your verticals. it provides a semi half circle to butt rightup against the longerons so when its painted there are no gaps and looks like a welded steel frame. thats a proctor device that's on the plans. the only change i made was to to add an upright of brass antenna to keep the hand drill on target and straight. i dont have a full size drill press just a dremel drill press and it wont take a 1/4" bit.here are a couple proctor pics of the device.