by a transverse opening 28 having a lip 30 positioned inwardly of the external surface of the sidewall a second horizontal recess 32 is formed along the inner surface of the slot 24 mounted in each of the key slots 24 are the spring 34 and clip 36 illustrated in fig4 spring 34 includes a substantially horizontal bight portion 38 having a slight bow therein as viewed from the top the bight portion aided by the bow is retained within the recess 26 at the upper end of each of the slots 24 the spring 34 also includes depending legs 40a 40b which are bent inwardly toward each other along most of their lengths but near their lower ends are curved out to provide camming portions 42a 42b the clip 36 is formed from spring steel it comprises a vertical back plate 44 which is normally substantially planar and is bent at its upper end to form an outwardly extending shelf portion 46 terminating in a downwardly extending flange 48 at each end of shelf 46 is a notch 50 52 the lower end of clip 36 includes a horizontal member 54 having a length substantially equal to the width of the key slot 24 and forwardly extending projections 54a 54b which extend forwardly a distance approximately equal to the thickness of the slot across the lower edge of horizontal member 54 is a rearwardly extending flange 56 approximately centrally located.