of a detent with nubbin 132 extending from handle 116 to rotate handle 116 into the retracted position as shown in fig4 handle 116 must rise to permit nubbin 134 to clear nubbin 136 such rise is accommodated by the compressibility of sleeve 132 handle 114 may be secured in the locked position shown in fig4 by a screw 138 penetrating a passageway 140 in the handle and into threaded engagement with aperture 142 of handle stop 144 see fig2 referring to fig6 there is shown a variant of handles 114 and 116 handle 150 is rotatably secured to rail 32 by a bolt or machine screw 152 penetrably engaging aperture 154 of the handle and into threaded engagement with aperture 122 in rail 32 a disc spring 156 is supported by bolt 152 and bears on handle 152 compressively as a function of the degree of tightening of bolt 152 thereby disc spring 156 provides resistance to pivotal movement of handle 150 similarly pivotally supported handle 160 includes a bolt or machine screw 152 penetrably engaging a disc spring 156 aperture 154 and threadedly engaging aperture 130 in rail 34 fig7 illustrates apertured plate 74 which includes a plurality of slots 170 172 174 and 176 through which devices 30 28.