The mark consists of a symbol consisting of a blue square with horizontal upper and lower sides, a white square superimposed on the blue square and having its corners meeting the middles of the sides of the blue square, a blue circular line centered inside the white square and touching the center of each of the edges of the white square, and two straight blue lines crossing each other perpendicularly at the center of the blue circular line, each straight blue line extending across the circular line and in parallel relationship with opposite sides of the white square.