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 fifth one made in 1932. The National Bureau
of Standards transferred it to the Smithsonian in 1957.
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.