Altimeter Type B-11 20K Ft, Air Force US Army

This is a WWII-era Sensitive Altimeter displaying aircraft altitude from 0 to 20,000 feet.  The Type B-11, made by the Bulova Watch Company, was used in a variety of aircraft during early WWII with a maximum performance ceiling below 20,000 ft. This altimeter has a serial number prefix of AF-43, indicative of a year of manufacture of 1943.  See spec sheet. 

Altitude was indicated from 0-10,000 ft by the long pointer along the outer scale. As the aircraft altitude increased beyond 10,000 ft, the higher digit was displayed by the small pointer along the inner scale up to 20,000 ft, while the longer pointer displayed the altitude in 1,000 ft increments more accurately along the outer scale. The knob adjusted the barometric scale inset at 3'o'clock on the instruments face to adjust for accuracy.

It is in exceptional condition given its age.  The knob turns smoothly, but is no longer accurate.