Sextant, Aircraft Periscopic, RAF Mk IIB, Ref No 6B/2838 Hughes The bottom knob controlled the

The bottom knob controlled the turn

It may be some sort of a stall warning instrument

which was indicated on the top gauge on the indicator

This is a Field Manual for the Identification of Japanese Aircraft published for US Military Intelligence by the War Department in March 1942

Sextant, Aircraft Periscopic, RAF Mk IIB, Ref No 6B/2838 Hughes The bottom knob controlled theHere is an original aircraft Periscopic Sextant Mk IIB as used in aircraft of the British Royal Air Force, in its original carrying case. The periscopic sextant was used in aircraft with a pressurized cabin, whereas the 'periscope' was inserted through a shutter mounted in the ceiling of the aircraft. This provided an unobstructed view of the sky for celestial navigation without compromising the pressurization of the crew cabin. The serial number of