Sextant, Aircraft Periscopic, RAF Mk II, Ref No 6B/2838

Here is an original aircraft Periscopic Sextant Mk II 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 this example denotes a year of manufacture of 1961, with certification in 1974. These were often carried aboard aircraft as emergency back-ups in these latter decades in the event of failure of the more sophisticated navigation computers that emerged in the 1960's and 70's. 

The case measure ~19 x 10x 12 inches.