There are provided interlock pieces extending from both edge portions of a side-to-side winglike piece provided in a pull-tab retainer toward an anterior wall part and a posterior wall part of the pull-tab retainer. An anterior support wall of an anterior mounting post and a posterior support wall of a posterior mounting post at superior angular corner regions of the internal faces thereof are provided with notches for interlocking of the interlock pieces. When the pull-tab retainer is fitted to the anterior mounting post and the posterior mounting post, the interlock pieces and the notches are interlocked with together to thereby prevent any displacement of the pull-tab retainer in anteroposterior side-to-side directions. Further, at that time, a side end face of the anterior support wall and a side end face of the posterior support wall can be covered up by the pull-tab retainer.