Compass, Type O.2.A., Ref 6A/892, with Azimuth Circle Ref 6A/890 with Original Case
Here is a WWII-era Observer's Compass, Type O.2.A. Ref 6A/892 with the mounted Azimuth Circle Ref 6A/890, in its original wooden case, as used in aircraft of the British RAF Coastal Command.
The O.2.A. is a table or pedestal mounted compass used by an aircraft observer. The compass bowl contains the rotating compass card which would be suspended in damping fluid. The azimuth circle is affixed to the compass body, and is released by unlocking (by turning) the toggle and sliding a small brass piece along its rim, which lifts a metal retaining peg from its hole. The circle is then rotated to place its glass mirrored prism toward a navigational target, such as the sun (with care taken to raise the darkened lenses). The direction of that target is then viewed through the prism, being reflected upward from the floating compass card. The text on the compass card is reversed so that it is legible in the prism mirror. The mounting bracket is located on the bottom of the compass. There is a small round bubble/spirit level on the azimuth circle to ensure that the compass is mounted on level.
This example is from the personal collection of RAF Wing Commander William James Hunter via his son, Dr. Allan Hunter. Wg. Cdr. Hunter (see photo) was trained as an RAF Observer, and responsible as a bomb-aimer, navigator, gunner, and radio operator. Wg. Cdr. Hunter and the crew of their Bristol Beaufort bomber were shot into the sea during an attack on the German battlecruiser Scharnhorst in July 1941, and was a POW for the duration of the war. His exploits can be read in his autobiographical account From Coastal Command to Captivity.
The compass measures ~6 inches diameter and ~6 inches tall. The case is ~9x9 inches and ~10 inches tall. The compass fluid has evaporated but it is otherwise in very good condition for its age. The compass card rotates as it should. The mirror in the prism is intact, and the card can still be read through it. The small bracket to the left of the prism is believed to be for a downward pointing lamp to illuminate the compass reading, which is not present. The label inside the lid has a printing date of Jan 1943.