Altimeter, Mk XIIIB, 0-20,000ft, British Royal Air Force

Here is a British Altimeter (Height Indicator) Mk XIIIB as used in pre-WW2 aircraft as well as aircraft during WW2, with a maximum ceiling of 20,000ft. It has the original crown symbol of the Air Ministry on the back.  The knob rotates the face of the instrument to correct the scale. According to the Royal Australian Air Force 346 Types and Locations of Instruments dated 1945, this indicator was used in the Avro Anson, Supermarine Seagull V, and Short Sunderland aircraft.