The Louden valves were made in the UK from 1924 by the Fellows Magneto company. The F1, F2 and F3 were all bright emitters. The F1 and F3 were both for detector and AF amplification. The F2 was for RF stages and had more turns on the grid and a higher AC resistance.
The box says that each valves carries the BBC Stamp and this one is the second type with the words Entirely British Manufacture. This exhibit is gettered.
The guarantee label and the connecting wires soldered to the outside of the split base pins.
The construction is unusual in the the anode is also a wire helix. The tungsten filament is held in place by having the supports clamped round it. In this exhibit the filament has burned out.
The envelope is 43 mm in diameter, and excluding the B4 base pins is 93 mm tall.
References: 1003 Type F1 was first introduced in 1924.