The VR21 is a battery low frequency triode as supplied to the Royal Air Force. The R indicates a receiving valve. When the Common Valve system was introduced in 1941 the VR21 was renamed the CV1021
The gettering is just thin enough for the shape of the anode to be seen.
Using the microscope and looking up from the base to inner structure.
The oxide coated tungsten filament wire is held at the pinch by crimping in a fold of the support.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 83 mm tall.
References: Data-sheet & 1040. Type VR21 was first introduced in 1929. See also1929 adverts.
