The VR21 (later called the CV1021) is a battery triode as supplied to the Royal Air Force. The VR stands for Valve Receiving.
The filament is in the form of an M with two tension springs at the top and a fixed support for the lower central loop. The grid extends to just outside of the bright anode.
The side view shows the front to back dimension to be around four mm.
The balloon envelope is 42 mm in diameter and, excluding the B4 base pins, is 87 mm tall.
References: Data-sheet & 1040. Type VR21 was first introduced in 1929. See also1929 adverts.
