74 has a width approximating the width of the lower rail 83 on central flange 76 the first tip 72 and the second tip 74 are of somilar design except that first tip 72 has an extension 84 which extends to cover the front face of body member 70 when first tip 72 is mounted on member 70 the first tip 72 and second tip 74 each have a semicylindrical barrel portion 86 and 88 respectively which fits into a matching semicylindrical receptacle on body member 70 to prevent tip rotation a pair of bolts 90 and 92 is adapted to extend through first tip 72 to secure that tip to body member 70 while a single bolt 94 is adapted to be passed through lower rail 83 to hold second tip 74 on body member 70 fig1 a and 13 b are cross sectional views of one of the second tips 74 the tip 74 has a semicylindrical body portion 88 rising from a flat back surface 100 and a pair of sloped walls 102 and 104 extending between back surface 100 and a rectangular front surface 106 tip 74 has a rectangular inner pocket generally designated 108 at the base of which sits a recess 110 the recess 110 has a profile matching the outer shape of a nut 112 adapted to fit onto the bolt.