generally t shaped having a stem portion 52 and an enlarged head 53 further an opening 54 is provided through the lower surface of each of the leg members 29 and 30 of the base member 16 adjacent the tip 37 such openings communicate through the lower surfaces of such leg members with each of the heel guide channels 35 and 36 in order to selectively lock the base member 16 into engagement with the side rails 17 and 18 a single elongated flat leaf spring 56 is disposed adjacent the rear 57 of the base member 16 and between the upper wall 31 and the lower wall 32 thereof the spring is mounted so as to extend outwardly through openings 58 and 59 through each of the side walls 33 of the base member 16 with particular reference to fig6 the length of the leaf spring is shown as being slightly greater than the spacing between the opposed serrated teeth 26 of the opposing side rails 17 and 18 therefore when the spring is in assembled relationship within the base member and contacting the opposing serrated teeth of the side rails 17 and 18 a slight bowing or prestressing of the spring is created with reference to fig4 in order to release or withdraw the ends of the locking spring from engagement with the teeth 26 of the side rails 17 and 18 a push button release member 60 is provided the push button release member includes an upper wall 61 having an upwardly offset portion 62 the release member is slidably carried within a bore or passageway 63 which is provided through and generally centrally of the rectangular portion of the base member to prevent the leaf spring from being displaced laterally of the base during assembly.