Navigational Computor Mk IIIH, Ref 6B/250, ICAN, RAF
Here is an British Mark IIIH Aircraft Navigational Computor (British spelling), Ref 6B/250, as used by pilots of the Royal Air Force during and post WWII.
Measures 5 inches by 6 inches and 1 inch deep. The knob scrolls the paper graphics beneath the clear lens. The ring around the lens rotates as it should, and the flight computer dial rotates.