leading edge of plate 12 left edge as shown in fig1 to the opening 19 and has an upright planar face 52 lying in a plane normal to the plate surface 13 and parallel to axis a guide member 50 extends from the opening 19 to the rear edge of plate 12 and has an upright planar face 53 also lying in a plane parallel to axis a and in coplanar relation to face 52 the guide members 49 and 50 are generally l shaped in cross sectional configuration and have base sections 55 and 56 respectively which are anchored to the upper surface 13 of plate 12 by screws 57 slidably attached to the face 52 of guide member 49 near the lead edge of base plate 12 is a bracket 58 which is adjustable downwardly towards or upwardly away from the plate surface 13 bracket 58 comprises a vertically disposed mounting section 59 that has therein a pair of parallel slots 60 fig2 for accommodating the screws 61 which secure the bracket to the face 52 of guide member 49 and a horizontally disposed nearly planar l shaped retainer section 62 that projects perpendicularly outwardly from face 52 parallel to the upper surface 13 of.