Rate of Climb Indicator, Mk IB*, 4000ft/min, British Royal Air Force
Here is a British Rate of Climb (Vertical Speed) Indicator Mk IB* likely Ref 6A/942, as used in RAF aircraft during WWII with a maximum rate of climb/descent of 4000 ft/min.
According to the document Royal Australian Air Force 346 Types and Locations of Instruments dated 1945, this indicator was used in the fighters Beaufighter, Spitfire, and Hurricane and bombers Mosquito and Lancaster. The serial number shows evidence of a year of manufacture of 1941 by KDG Instruments. It has the original crown symbol of the Air Ministry on the back. See image of the Spitfire 16 cockpit panel from its Pilots Notes showing the location of the indicator.
This indicator is in good condition given its age. The knob would rotate the pointer to zero it in flight, but is immovable on this example. Measures ~3.75 inches diameter and ~2 inches deep.