carried on a generally rectangular body 34 and is mounted in each channel guide 33 the tab 32 has a smoothly rounded tailing edge 35 and is of generally the same thickness as integral collar 23 extending outwards from the tab 32 the rectangular body has a ramp face 36 that slopes upwardly as best seen in fig3 a the tab 32 is movable in the axial direction within the channel guide 33 and is upwardly biased by a compression spring 37 which is placed between the bottom wall of channel guide 33 and the bottom of body 34 surrounding the outer periphery of the middle portion of body 12 and adjacent the ring 30 and the integral collar 23 is an annular ring that forms a rotatable collar 38 the collar 38 is preferably made from a plastic or similar material as best seen in fig3 a and 7 to 9 the collar 38 has two diametrically opposed cavities 39 opening from its inner cylindrical surface and each receiving one of said tabs 32 with clearance so as to permit limited angular movement of the collar 38 circumferentially around the body 12 and between what will hereinafter be referred to as an inoperative position and an operative position within each cavity is a tension spring 40 connected at one end to a pin 41 formed on collar 38 and at the other to a pin 42 located adjacent the corresponding channel guide 33 on body 12 the spring 40 resiliently biases the collar 38 in the inoperative position each cavity opens to the top face of the collar 38 through a slot 43 that is aligned radially with the upper edge of ramp face 36 in each cavity 39 there.