Jet computer, Type B.26. Three concentric rotating metal scales on each face secured together with central screw, with apertures on the central scale of each face for additional scales.
Obverse: main dial carries inscription “Jet Computer for navigation. Dead reckoning”.
Outer scale: True Air Speed, miles.
Middle scale: Minutes & hours, Mach number
Inner scale: Pressure altitude
Apertures: Density altitude, Speed, fuel 100 - 900.
Reverse: Outer scale: Wind Comp.
Middle scale: Wind, True airspeed, Latitude.
Inner scale: Drift left, Drift right.-
Aperture: Drift (in degrees)
Purpose: the computer allows true airspeed, ground speed, drift, course, Mach number (the ratio of actual speed to the speed of sound at that altitude), to be found from knowledge of indicated airspeed, wind direction and speed, air temperature. The scale ranges are appropriate for high speed (jet) aircraft operating at high altitudes (35,000 feet). Fuel calculations are also provided for.

There is an accompanying instruction booklet.