<p>A pair of latching pieces (24) is formed in the opening (23) of a pull tab (20), and a pull tab pivot support (14) that supports the rotational pivot (21) of the pull tab (20) is covered with the narrow part (3b) of a cover (2) that is affixed to the upper plate (11). The plan view shape of the wide part (3a) of the cover (2) from the front end to the pull tab pivot support (14) is configured to have the same shape as the plan view shape of the upper plate (11) in the same region. An engaging part (4) that is engaged between the pair of latching pieces (24) extends from the narrow part (3b). The upper surface (3) of the cover (2) can thereby be configured as a flat surface and the disengagement of the pair of latching pieces (24) from the engaging part (4) can be accomplished with a moderate operating force.</p>
