thereto a cylindrical bushing 104 with an eccentrically positioned circular opening 106 extending therethrough a flat washer 108 and a nut 110 each cam roller assembly 98 is attached to its associated support rail mounting boss 96 by positioning the bushing 104 in the opening of the boss 96 extending the stud 102 outwardly through the bushing opening 106 and then securing the washer and nut 108 110 to the outer end of the threaded stud 102 this positions the roller member 100 at an associated one of the support rail member cutout areas 84 in a manner permitting the roller member 100 to be laterally adjusted to cause a side portion 100 a of the roller member 100 to project a selectively variable distance inwardly through its associated cutout area 84 and past the inner side surface of its associated support rail member see fig6 and 13 such adjustment of the roller member 100 relative to its associated support rail member is achieved by appropriately rotating the roller assembly boss 104 in the opening of its mounting boss 96 and then locking the rotationally adjusted boss 104 in place using for example a set screw not shown this adjustment of the roller members 100 as best shown in fig1 laterally shifts their rotational axes 112 selected distances.