This is a small left-handed octant with a Willson bubble telescope, a Fischer
rapid release lever, a drum micrometer, an electric light over the divided arc,
and a battery in the handle. The Navy Bureau of Aeronautics designated it the
Mark I, Model 3. The Naval Observatory transferred this instrument to the
Smithsonian in 1963. It was the 39th example of the Model 206B, and the 19th example made for the Navy in 1929.
One advertisement noted that the left-handed octant was designed "so as to leave the right hand free to write down the time and the observed altitude." Another noted that "It is indeed amazing that nobody ever thought before of making a sextant for the left hand so as to avoid the necessity of putting down the instrument or changing hands every time an observation had to be noted. The change of design from right to left was first proposed four years ago by the Bureau of Aeronautics of the United States Navy, and it has now met with general approval."
Ref: R. W. Willson, "Instrument Including Means for Determining the
Horizontal Direction," U.S. Patent #1,705,146
Pioneer Instrument Co., Pioneer Instruments and Equipment (Brooklyn, ca. 1930).
Pioneer Instrument Co., Pioneer Marine Instruments of Aircraft Quality and Precision (Brooklyn, 1930), quoted in F. M. Rogers, Precision Astrolabe: Portuguese Navigators and Transoceanic Aviation (Lisbon, 1971), p. 179.