This valve from Mullard has the identification LF and matched the HF Red Ring in design.
Alternatively it could also be a DE06LF from about 1925 that was the dull emitter version of the LF.
The LF is painted with the single green ring and is an AF triode. The type DE06HF had a red band and was the dull emitter version of the HF.
The balloon envelope is 34 mm in diameter and, excluding B4 base pins, is 85 mm tall.
Reference: 1003. Type LF was first introduced in 1924.
Absolute Maximum Operating Conditions¶
Updated April 20, 2013. shape:balloon construction:pinch type:triode age:1920.1930 base:b4 heater:4.0v pins:4 pin:1.a pin:2.g1 pin:3.f pin:4.f