l or 82 r are provided which compose one side of the support portion 8 l or 8 r the rotation supporting concave portion 83 l or 83 r into which a rotation supporting convex portion 81 l or 81 r composing the other side of the support portion 8 l or 8 r is fitted is formed in the front side of the fitting member 6 l or 6 r and in an oval shape which is elongated in the front and rear direction further the rotation guiding concave portion 82 l or 82 r into which a rotation guiding convex portion 84 l or 84 r composing the other side of the support portion 8 l or 8 r is fitted is formed in the rear side of the fitting member 6 l or 6 r the upper side of the rotation guiding concave portion 82 l or 82 r is opened from the upper end edge toward the lower side thereof and the length of the rotation guiding concave portion 83 l or 83 r in the front and rear direction is set to almost the same length as that of the rotation supporting concave portion 83 l or 83 r in the front and rear direction the fitting member 6 l or 6 r has a plurality of convex portions 91 l or 91 r formed in the rear end portion thereof along the vertical direction the convex portions 91 l or 91 r composing one side of the ratchet mechanism 9 l or 9 r reference numerals 85 l and 85 r represents a guide plate which guides the rotation of the fitting member 6 l or.