This is the original model of the Spherical Marker, and still the most popular and economical. It was designed in the 1960s in cooperation with the Arkansas Department of Aeronautics and Arkansas Division of the Federal Aviation Administration. It has remained in continuous production, with only minor changes, since its introduction. The SD Marker installs easily on any wire .2" - .875" through the use of two UV resistant neoprene strip bushings installed on the wire to match the diameter of the openings (boss) in the Marker. The Marker is then securely bolted around the wire, clamping down the bushings.