The holder has a cylindrical main body portion having an outer cross section that is equal to or smaller than a cross section that defines the inner wall of the upper portion, and the main body at a pair of slot positions with the main body portion positioned within the inner wall of the upper portion when viewed from the central axis direction. And a pair of locking pieces projecting outward from the front end in the central axis direction of the portion along a direction perpendicular to the central axis, and the pair of locking pieces are positioned in the pair of slots and the head portion The engaging portion engages with the side wall connecting the outer wall and the inner wall and / or the rod member held in the pair of slots, and the pair of locking pieces are positioned in the pair of slots and It has a shape that fits within the outer shape.保持具は、上側部分の内壁を画する断面以下の外形断面を有する筒状の本体部と、中心軸方向から見て本体部が上側部分の内壁内に位置した状態で一対のスロット位置において本体部の中心軸方向先端部から中心軸に垂直な方向に沿って外方へ張り出した一対の係止片と、を備え、一対の係止片は、一対のスロット内に位置した状態でヘッド部の外壁と内壁とを繋ぐ側壁および／または一対のスロットに保持されるロッド部材に係合する係合部を有し、一対の係止片は、一対のスロット内に位置した状態でヘッド部の外形内に収まるような形状を有している。
