This instrument resembles the "Mate" sextant made by Henry Hughes & Son, Ltd., of London. It has a blackened brass frame, and a brass scale that is graduated every degree from -5o to +135o and read by drum micrometer to 30 minutes of arc. It was examined at the National Physical Laboratory at Teddington, England, in 1942.
The DS&S insignia refers to David Shackman & Sons, the British firm that actually made this sextant. The referenced patent (GB556034), issued to Albert Shackman and Reuben Cyril Shackman in September 1943, describes details of the drum micrometer.
Ref: Kelvin & Wilfrid O. White Co., Nautical Instruments (Boston, 1940), p. 31.
Henry Hughes & Son, Ltd., Husun Navigational Instruments (London, 1939), p. 31.
M. V. Brewington, The Peabody Museum Collection of Navigating Instruments (Salem, 1963), p. 132.