The VR21 (later called the CV1021) is a battery triode as supplied to the Royal Air Force. The R indicates a receiving valve.
The up arrow followed by a Z indicates a valve made at the Hammersmith works of M-OV.
Without the metallising the bright electrodes are easily seen. At the top are two helical springs with hooks to hold the filament strand in tension so that it does not sag when hot and change the operating characteristics.
A closer view.
The detail of the spring - taken with the microscope and thus the image has a very small depth of field.
The balloon envelope is 42 mm in diameter and, excluding the B4 base pins, is 82 mm tall.
References: Data-sheet & 1040. Type VR21 was first introduced in 1929. See also1929 adverts.
