Turn and Slip Indicator, Mk IA, 6A/675, RAF 1939 Core

Here is a Turn and Slip Indicator, Mark IA, Type 6A/675, invented by George Reid (according to the 'all-knowing' Wikipedia) and made by Reid & Sigrist, as used on the blind flying panel of aircraft of the British Royal Air Force and the Royal Australian Air Force of WWII.  It is an early version as evidenced by the all-white markings; later versions had both white and green markings on the face. 

While the Mark IA was used across numerous British aircraft during the war, including the Spitfire, Hurricane, Beaufighter, Mosquito, Sunderland, Lancaster, Battle, and Anson, this indicator appears to have a production year of 1939 (4389/39), so it could have been from early versions of these or other aircraft.  Measures ~4.25 inches diameter and in depth. 

This example is in need of a glass lens, a lens gasket, and outer retaining ring.  The threads that would capture the retaining ring may require filing for a smooth fit.