knob 40 so as to extend over the substantially entire length of the inner member 10 the rod support mechanisms 16 may be formed in the upper surface of the inner member 10 at a certain interval in the front back direction i e the forward and backward direction in order to slidably support the rod 50 the rod support mechanisms 16 may allow the rod 50 to move or reciprocate back and forth further the rod support mechanisms 16 may each have the substantially same structure the rod biasing mechanism b may be formed in the upper surface of the inner member 10 the rod biasing mechanism b may preferably be positioned between the rod support mechanisms 16 the rod support mechanisms 16 and the rod biasing mechanism b may preferably be aligned with the manipulation knob 40 in the front back direction the latching mechanisms l la and lb may respectively be positioned in a front left corner and a rear left corner of the inner member 10 as shown in fig6 the manipulation knob 40 may have a shaft portion 42 projected backward from a knob body and a pair of detent projections 44 formed in the knob body as shown in fig9 the shaft portion 42 may penetrate a rear wall 14 a of the knob receiving recess 14 and project toward the rod 50 therethrough further the manipulation knob 40 may have a first biasing member or return spring compression spring 32 that is attached to or journaled on.