not required the slots 33 and 34 receive portions of the retainer spring clip 35 as shown in detail in fig3 and 5 a flanged end 40 of the clip 35 extends through the rear panel 30 at aperture 40a to provide a stop against which the spring clip 35 can be tensioned the clip 35 has a first curved portion 41 which abuts a portion of the top 19 between the rear panel 30 and ridge 32 a second curved portion 42 extends over the ridge 32 and joins a third curved portion 43 which extends into and bears against an edge 33b of the slot 33 a fourth curved portion 44 of the clip 35 extends over the ridge 32a and joins a retaining means for holding the clip 35 in place comprising a short horizontal portion 45 which is held by spring tension immediately beneath and parallel to the top 19 in slot 34 the section 45 joins a straight section 46 which terminates in a curved flange 47 a suggested method of disengaging the clip 35 from the slots 33 and 34 in the top 19 is shown in fig4 the blade 50 of a screw driver or other suitable tool is inserted in the flanged end 47 of the clip 35 and a.