The Mark IV, Model 1 was an improved version of the Mark I, Model 2–lighter
and more compact, and with the divided scale on the inside along with the
optical train. This example was the third one made in 1932. The U.S. Naval
Observatory transferred it to the Smithsonian in 1963.
Ref: T. C. Lonnquest, "Operation and Adjustment of the Mark II Model 2 Octant," Bureau of Aeronautics, Navy Department, Technical Note No. 207 (Nov. 22, 1929).
Lawrence Radford and Noel Davis, "Sextant," U.S. patent #1,743,979.