The CV1021 was originally called the VR21 and is a battery triode for LF use.
The KB/T identifies this valve as being made to specification K1001 or K1006, the B denotes qualification by a UK authority and the T identifies the maker as British Tungsram, Tottenham.
The small triode is mounted well above the base pins and the extra lead inductance will limit the HF use.
The oxide coated filament is in the shape of an M with two tensioning springs in place above the top mica.
At this angle the helical springs with their hook ends can be seen holding the filament. The triode geometry is also visible.
