The valve looks like a late (1960s) specimen of the pre-war American Type 211. Valves of this type were at one time used professionally as audio modulators or sub-modulators.
This valve was originally used by Radio Newcastle.
It is a directly heated power triode with a special four pin base.
The design purpose was as a high power audio amplifier.
Updated April 24, 2013. shape:wide.glass.tube construction:pinch type:triode base: pins:4 pin: