The 6021 is a special quality sub miniature double triode. The UK Government procurement code of CV3986 is also on the glass. The double mica discs top and bottom are present to prolong h life of the valve in operational areas of mechanical shock and vibration.
The ultra miniature triodes within the sub 10 mm glass envelope.
The cathode tubes are rectangular with rounded ends. the getter holder can be seen as a ring on the left.
The thin glass tube envelope is 9 mm in diameter, and excluding the B8D/F base wires is 35 mm tall.
