Compass, Telltale Mirrored Magnetic RAF Type P-12 6A-1673 (COMP16)
This RAF telltale compass, intended to be installed upside-down in an aircraft, is pre-WWII.
According to collector George Fulford's notes:
Old salts will appreciate this rare one. Skippers of long ago would have a telltale compass hung over their bunks that could be read upside down so that he would be well aware if the quartermaster was sleeping at the wheel.
The RAF liked the concept and they incorporated these modified compasses to fit into those 1930 bombers like the Hampden that were very narrow in the lone pilot’s cockpit. It also had the advantage of being further away from a lot of deviation inducing equipment.
So, it was screwed upside down and angled the large square mirror so you could read the compass as clearly as the standard one. I’ve checked it out and it’s marvelous how effective it is.