20 upon further rotation of the loading ring 1 the cam 41 drops on the camming surface 46b as shown in fig3 c the opened portion of the cam 41 is narrowed so that the arm 20b of the second guide post unit 20 is called in to the catcher 42 side by the camming surface 41b in such a manner a preliminary catching is performed at the same time an upward slant portion 27e see fig5 a of the front end of the hook 27 within the second guide post unit 20 rises by its own resiliency along a downward slant portion 44d not shown in fig3 a provided as a unit with the guide 44 and rides on a flat portion 44e see fig5 b therefore the hook 27 is disengaged from the extension 3 of the loading ring 1 permitting the loading ring 1 alone to continue rotating in the direction of arrow a as the loading ring 1 further goes on rotating when the first guide post unit 10 reaches the position of the first catching structure 30 as shown in fig3 d the first guide post unit 10 contacts on the first catching structure 30 and is caught the loading ring 1 continues rotating until the protuberance 2 moves away from the hole 11 while further charging the spring 49 to exert a pressure of the direction indicated by an arrow e in fig3 d the catching of first guide post unit 10 is ensured by its force also the second guide post unit 20 is pushed deeper and pressed on the catcher 42 by the rotation of the loading ring 1.