Ports Required – There shall be provided for each motion picture projector not more than one (1) projection port, which shall be limited in area to 750 sq. centimeters, and not more than one (1) observation port, which shall be limited in area to 1,300 sq. centimeters. There shall be not more than three (3) combination ports, each of which shall not exceed 750 millimeters by 600 millimeters. Each port opening shall be completely covered with a pane of glass; Except, that when acetate safety film is used, projection ports may be increased in size to an area not to exceed 4,500 sq. centimeters.