Altimeter, Mk XIIIC, 30,000ft, British Royal Air Force Ref 6A/445
Here is a British Altimeter (Height Indicator) Mk XIIIC Ref 6A/445 as used in RAF aircraft during WWII with a maximum altitude of 30,000ft. The knob rotates the face of the instrument to correct the scale, which is then indexed by the small round window at the 5 o'clock position. Altitude is displayed along the outer scale as the pointer rotates clockwise from 0 at the 6 o'clock position to 16,000 ft at the 5 o'clock position, and then from 16,000 to 30,000 ft along the inner scale.
According to other websites which showcase British aviation artifacts, this altimeter was used in early Supermarine Spitfire fighters. According to the Royal Australian Air Force 346 Types and Locations of Instruments dated 1945, this indicator was also used in the Fairey Battle light bomber. The serial number shows evidence of a year of manufacture of 1939 by S.Smiths & Sons (S.S. & S), Ltd. It has the original crown symbol of the Air Ministry on the back.
This altimeter is in exceptional condition given its age. The knob rotates and the card turns as it should. Measures ~3.75 inches diameter and ~2 inches deep.