SD stands for Special Dial for Seiko watches that boast indexes made from solid 18K or 14K. There was also another variant of index enhancement called AD or Appliqué Dial where indexes are made from Gold or Rhodium plated on brass. These logos are still in use today, as seen in the Grand Seiko Kintaro Hattori 160th Anniversary Limited Edition SBGZ005 and Seiko 140th Anniversary Limited Edition SBGW260.

Photo : Plus9Time