the case may be will enable alignment of the v groove 31 with regard to the centerline 22 drawn along the edge of the board 20 the purpose of having this adjustment of course is to make possible drilling of holes in the board edge in absolute centrally disposed locations to that end the outboard block 30 is provided with certain drill bushings such as 37 of identical form which have flanges at their upper ends designated 38 in each case to position the drill bushings 37 in their desired location in the block 30 the bushings 37 are of any preferred inner diameter to receive drills of corresponding diameter for the drilling operation and the bushings in turn are equipped with certain detents such as indicated at 38a in each case these detents prevent rotation of the bushings while the drilling operation is being performed in either one or the other of the bushings 37 these may be described as sort of ramp like formations which are formed by an appropriate tool arranged to strike the flange 38 to thereby cause a portion of the flange to be raised above the plane of the flange each detent is arranged as a protrusion from the upper surface of the face of the flange 38 to engage a central stop screw 40 which is mounted in the block and thus prevents rotation of the respective bushings during the drilling operation the flanges 38 of the bushings are provided with suitable sliced off areas 38c to permit removal of the bushings when.