The RAF used a number of electrical instruments in its aircraft and ground crew had a supply of spare instruments to replace damaged or faulty instruments. One of those instruments was the milliammeter, used to measure electrical current. A milliamp is 1000th of an amp, so a very small current. This spare milliammeter is still in the cardboard box that it came from the manufacturer in:

The label on the box shows that this was manufactured by Ferranti Ltd, a major electronics company that had been founded in 1885. During World War II, Ferranti became a major supplier of electronics, fuzes, valves, and was, through development of the Identification Friend or Foe (IFF) system, heavily involved in the early development of radar in the United Kingdom

Opening the box, the instrument is packaged in corrugated card to keep it safe from damage:

The milliammeter is marked up as belonging to the Air Ministry with the initials AM either side of a crown. The date of 194 is marked in the bottom corner, although the final year digit has not been added:

A pair of terminals are fitted to the rear, with the positive terminal clearly marked so that the instrument can be correctly wired up: