the same root reference numerals will be utilized to describe the same structure in each leg the orientation of the legs in the drawings can appear different depending upon the placement of a movable latching member 32 which will be described hereinbelow as will be seen latching member 32 may be placed in either of the legs depending upon the desired requirement first leg 12 includes a first side wall 16 and a second parallel side wall 18 first side wall 16 has a greater height than second side wall 18 which has a curved side wall portion 20 to extend upwardly to a top wall 22 top wall 22 includes a leaf spring 24 with an aperture 26 formed between leaf spring 24 and a wall at the rear of the first leg 12 and second leg 14 there is provided a transverse wall 28 which extends between first side wall and second side wall 18 an aperture 30 is formed in transverse wall 28 for reasons which will become apparent hereinbelow the corner includes a movable latching or retaining member generally designated by reference numeral 32 movable retaining member 32 has a front wall 34 and a rear wall 36 a pair of side walls 38 40 extend between front wall 34 and rear wall 36 at one extremity there is provided a first sloping top wall 42 and a second sloping top wall 44 which.