A two-dimensional code that includes a bar code readable by a scanning operation is provided. This filter for an image display device is excellent in contrast and color reproducibility of an image and also in durability, and is suitably used for a plasma display panel. The lithium-cobalt-manganese oxide synthesized according to the present invention has a stable and superior electrochemical characteristic. An equalization reservoir is connected with the damper cylinder such that the damper fluid is provided for volume equalization of a time-variable displacement of the damper fluid by the damper piston rod upon spring deflection and rebound of the telescopic strut. The method includes: directing a laser beam onto a material attached to the mask; and removing the material attached to the mask.