disposed in a common plane parallel to and spaced from the panel 31 these walls 30a 30b and 32 form an inverted u shaped rim which partially encircles a central tongue receiving cavity 35 the central portion of the end wall 32 is recessed or notched to define an inwardly opening channel or groove 36 which cooperates with a similar channel of an opposed registering element 29 to form a closed tongue receiving aperture a concaved upperwardly opening recess 37 centrally in the top end 32 forms a finger access opening inwardly of the tongue receiving groove 36 the cavity 35 is provided with a plurality of staggered right prism or rectangular tongue receiving blocks 40a 40b 40c 40d and 40e disposed in opposed parallel vertical rows each block being integrally joined by its inner end to the inner surface 31b of the panel 31 blocks 40a 40c and 40e which are inwardly adjacent to wall 30a have inwardly opening recesses or apertures 41a 40c and 41e which are in vertical alignment with each other along one side portion of the cavity 35 blocks 40b and 40d which are inwardly adjacent to wall 30b have inwardly opening recesser or apertures 41b and 41d in vertical alignment with each other along the other side of cavity 35 blocks 40a 40c and 40e are respectively vertically spaced from each other by a distance approximately equal to the respective widths of blocks 40b and 40d also the blocks 40b and 40d are spaced apart by a.