being horizontal the fastening device 137 also possesses a wire fastening element designated as a whole by the reference 147 a gripping element and in this example composed of a wire made of an elastic material this wire 147 is formed in the manner of a hairpin the branches of which are bent perpendicularly relative to their plane so as to define a groove which comprises a first part bearing against the front face of the plate 138 a second part bearing against its rear face and a third part bearing against its end face and in which the lower part of the plate 138 is gripped the wire 147 comprises a head 148 forming the first part of the gripping wire 147 bearing against the outer face 149 of the plate 138 two branch parts 150 forming the third part of the gripping wire 147 bearing against he lower horizontal end face 143 of the plate 138 and inclined in the plane of the end face 143 relative to the transverse direction of this end face 143 at the same time opening and in the extension of these branch parts 150 two branch parts 151 which extend vertically and upwards.