A window is formed in a common use cover for receiving one or a plurality of wiring boards therein, the inside and outside of the cover communicating with each other through the window. A convex portion for enabling the kind of the wiring board to be distinguished is formed on each of the wiring boards, and when the wiring boards are properly received within the cover, the convex portions project through the window to the exterior of the cover, or are disposed in facing relation to the window, and when the wire boards are improperly received in the cover, the convex portions are interfered with a wall of the cover. Each of the convex portions has an obverse/reverse distinguishing-function portion for enabling the obverse and reverse of the wiring board to be distinguished.