end 113 bears against the bridge member 114 from behind and which preferably is disposed a short distance above the spring base portion 115 which rests upon spaced ribs 120 to permit the leg 110 to deflect downwardly therebetween the base portion 115 has two supporting tongues 116 extending beyond the finger 112 to bear against the end wall 117 of the recess 118 the leg 119 has two struck out tabs 120 which serve as claws with the metal therebetween deflected downwardly to form a nose 121 at the bottom 122 of retaining pocket 118 there is a central groove 123 which permits the resilient finger 112 to deflect when being inserted in the retaining recess 118 beneath the bridge member 114 in the embodiment shown in fig4 to 4b the central portion of the spring strap 130 also bears against the underside of the bridge member 131 while the retaining leg 132 is braced against the cover flange 133 at the bottom of the recess 134 at both ends of the spring strap 130 there are curved sections 135 and 136 directed toward the flange 133 and bearing the legs 132 and 137 respectively as illustrated in fig4 b the retaining leg 132 consists of two spaced fingers 138 which rest flat on the bottom 133 and are directed from the curved section 135 toward the curved part 136 the spring strap 130 which bears against the bridge like part 131 has a struck out retaining tongue 139 which slopes upwardly and catches behind the bridge member 131 to prevent the resilient fastening clip 140 from being pulled out of the recess 134 the radius of the curved piece 136 is so chosen that when installed as shown in fig4 it does.