The makers put a lot of information on the box but as the top of the box is missing we do not have the full type designation. The dome of the envelope has a clear letter F but the first letter is missing.
The base pins are an unusual form of spring construction.
The box makes great play of the British nature of the manufacturer.
The balloon envelope is 42 mm in diameter, and excluding the B4 base pins is 90 mm tall.
Reference: Observation. Type LF was first introduced in 1924.
Updated April 12, 2013. shape:balloon construction:pinch type:triode age:1930.1940 base:b4 pins:4 pin: