omitted thereof the third embodiment according to the presented invention is shown in fig3 the ferrule 1a fig3 a is of a hat like configuration having an upward and a downward part both parts are of horizontal flat type contrary to the ferrule 1 of the first and second embodiments as shown in fig1 a the ferrule 1a and upper notch 2b are made of rigid material in the central opening 12a of the ferrule 1a the inner wall surface provides a ring like groove or stepped diameter area 121a for receiving a flexible ring 5 as shown in fig3 b in order to accomodate to the structure of the ferrule 1a the upper notch 2b is constituted of a projecting part 21b having a lower section 211b a middle section 212b and an upper section 213b the outer diameter of the lower section 211b equals that of the central opening 11 of the ferrule 1a but is larger than the inner diameter of the protruding flange 51 in the flexible ring 5 the upper section 213b has an arc and taper configuration and its largest outer diameter is a little bit bigger than the inner diameter of the protruding flange 51 the diameter of the middle section 212b is smaller than the upper and lower sections 211b 213b but corresponds to or is smaller than the inner diameter of the protruding flange 51 the principle of coupling between the upper notch 2b and the ferrule 1a according to this embodiment is similar to that of the second embodiment as shown in fig2 except that the female and male organs in the upper notch and the.