gusset or a rib 114 fig6 b is attached perpendicularly to and extending between surfaces 102 and 90 having sufficient thickness and length so as to resist flexing stresses between plate 100 and clamp 88 a pivoting cam tensioning clamp 118 comprised of a top surface 120 a parallel bottom surface 122 a generally perpendicular front surface 126 connecting surfaces 120 and 122 and a angled upper back surface 124 and a angled lower back surface 125 connecting to surfaces 120 and 122 respectively top surface 120 is interrupted by a elongated cam receiving channel 98 b which extends fully along the length of surface 120 a pivot 130 is attached to pivoting clamp 118 and has its center point located at the intersection of surfaces 92 and 102 with the pivot longitudinal axis parallel with the longitudinal axis of fixed cam channel 98 a and 98 b pivot 130 also attaches to cam index clamp 88 with the center of pivot 130 located at the intersection of surfaces 92 and 102 thus it is seen that when fixed cam index clamp 88 is attached to pivoting cam tensioning clamp 118 via pivot 130 elongated cam receiving channels 98 a and 98 b are situated directly opposite each other and cooperate to form a elongated cam cavity 132 fig6 a elongated cam cavity 132 being of a predetermined size to accept elongated cam 82 within a coil spring 134 is positioned to exert tension onto pivoting cam tensioning clamp 118 a tensioning screw 136 passes through spring 134 and through clearance hole 128 in pivoting clamp 118 and threads into a.