Cal 4826 (marked as 4826-900-M1) Serial number 721655 which dates the watch to February 1977. Suwa maker above the 6 maker. Produced only for the Japanese market. There are 12 segments of gray crystal silicon cells that surround the face. The face is black with white hour markers separating the 12 segments of solar cells.
Comment: The Citizen Crystron came out a year before this Seiko model making it the first analog solar watch. However, the design of this Seiko better integrates the solar cells into the face design with the choice of colors and layout.