the front as shown in fig8 10 or an oblique surface 52 as shown in fig5 for adaptation to the shape of the front edge 1 39 of the lid it may also include a convex surface in the embodiment shown in fig5 a pivot comprising two journals 53 and 53 39 is disposed on the back of the top beam the journals are separated by a flattened surface 57 the journals 53 53 39 are connected on the side of the central leg to the control bar 51 by a continuous inclined surface on the opposite side they are connected via an oblique plane 54 and a plane 55 that is substantially parallel to the central leg of the t a throat 56 that is substantially parallel to the bar of the t is located between the plane 55 and the two journals 53 53 39 the two journals 53 53 39 are fixed to the bar of the t by a surface that is as small as possible but nevertheless is sufficient to assure a suitable mechanical resistance in another embodiment the pivot does not include the flattened surface the central leg of the t comprises a latching prong 58 substantially disposed along the axis of symmetry.