the clip 20 for receiving a portion of the pivot ring 30 the hook 28 as seen in fig4 and 5 has a width measured side to side which is less than that of the remaining portions of the clip the central portion of the arm 22 intermediate to the stop 24 and bottom projection 26 has a generally rectangular cross section with a width that is greater than its thickness the bottom portion of the clip that includes the projection 26 has a generally square cross section except at the hook a lip 29 is formed at the tip of the hook 28 the lip is mechanically deformable as best shown by fig6 the cap 10 is a tubular body in the form of a circular cylinder preferably a thin metal cylinder the cap 10 has an axial bore 13 longitudinally extending between two opposite open ends lower end 12 and an upper end 14 at the end of a tapered frusto conical upper end portion 16 of the cap a longitudinally extending rectangular slot 15 adjacent to the upper end portion 16 extends from the outside surface through the side wall of the cap 10 to the inside surface the slot 15 is sized to receive the bottom projection 26 and the hook 28.