This is a small left-handed octant with a Willson bubble telescope, a Fischer
rapid release lever, a drum micrometer, electric light over the bubble and
divided arc, battery in the handle, and radium illuminator for star
observations. Since the legs are on the front of the frame, the octant can be
set down with the handle uppermost. The Navy Bureau of Aeronautics designated this the Mark I, Model 4. The pending patent has not been identified.
This was the 134th example of the Model 206C, and the 40th example made for the Navy in 1931. The Navy transferred it to the Smithsonian in 1963.
Ref: R. W. Willson, "Instrument Including Means for Determining the
Horizontal Direction," U.S. patent #1,705,146
Pioneer Instrument Co., "Aircraft Octant, Type 206 with Willson Bubble Telescope, Instructions," and "Appendix to Instructions for Octant Mark I, Model 4."