provided on the first half 40 of the said plate the plate 30 g which is provided in the lowermost position does not comprise any tongue but features further to aeration and lightening holes 41 and to sets of three hooks as illustrated earlier in this description a tapered receptacle 57 for the lowest end portion 28 of the longitudinal connection element 20 see fig3 which illustrates how the various plates 30 a 30 g are assembled together the tongues 52 a 56 b of any plate come to lie underneath the upper half 40 of the subsequent plate and increasingly spaced from the latter as one moves away from the rib 32 towards their free end portion as this is also illustrated in fig4 and 7 it therefore ensues that all over the height of the plate assembly 30 except for the upper half of the plate 30 a situated in the uppermost position there are two overlapping plates that are spaced from each other with the possibility of moving in all directions with respect to each other fig7 further shows that it is optionally possible for a thermoformed or punched strip 70 of a soft material such as for instance a.