the wall the height of the shield 34 is greater than the height of its corresponding opening so that the shield overlaps the opening the shield 34 also has two spaced tabs 34 c and 34 d formed along one edge portion which is its lower edge portion as viewed in fig3 with a notch 34 e being defined between the tabs two recessed portions 34 f and 34 g are also formed along the upper edge portion of the shield 34 for reasons to be described the shield 36 is identical to the shield 34 and as such has two snap tabs 36 a and 36 b formed at each end thereof which function in the same manner as the snap tabs 34 a and 34 b of the shield 34 the shield 36 also has two tabs 36 c and 36 d and a notch 36 e formed along its lower edge portion and two recessed portions 36 e and 36 f formed along its upper edge portion the shields 34 and 36 are shown in their interlocked position in fig4 in this position the tabs 34 c and 34 d of the shield 34 overlap the upper edge portion of the shield 36 and extend in the recessed portions 36 f and 36 g of the shield 36 also the notch 34 e of the.