clarity after emplacement the securement of this return spring 11 takes place by insertion of its front end into the slot 13 a of the floor 13 and bending this front end to form a holding hook to balance the pressure of the spring 11 each pusher strip 8 is positioned by its ends 14 a into hollow portions 1 a of the front rail 1 spaced by about the width of the floor 13 whilst its snap in tenons 14 b are also adapted to be snapped into configurations 7 a 7 c forming the mortise of the front rail 1 spaced by a width about the width of the floor 13 the front end 4 of the separation strip 3 is shown without its flaps 16 a 16 b of flexible material and has a single positioning lug 4 a in a corresponding hollow 1 a of the front rail 1 and a single tenon 4 b for snapping into a mortise 7 b of the front rail 1 thus after having engaged the forward heads either of a pusher strip 8 or of a separation strip 3 there can be carried out a pivoting until the wall 14 c or 4 c comes into abutment against the wall 1 b of the front rail 1 while being ensured a perfect perpendicularity of a strip 3 or 8 forming a cross piece relative to the front rail 1 because of the pivoting that takes place in a plane perpendicular to the front rail 1 all the strips 3 or 8 positioned relative to the front rail 1 are thus parallel and the rest after snapping in which.