The B4 was the first dull emitter valve from BTH (The British Thomson-Huston Company) and was first introduced in 1923. In 1924 the B6 was released that reduced the filament consumption but offered about the same performance. BTH claim that the B4 was the first 6 Volt valve in the UK to draw only 0.25 A.
The design of the B4 was for an AF amplifier to drive small loudspeakers. These would not be the moving coil devices of today but probably a horn device powered by an oversize earpiece and being wound with a high impedance coil that was wired directly into the anode circuit of the valve.
The base is plastic and the valve carries the final type of BBC stamp and so dates to between mid 1924 and the end of 1927.
The silver gettering covers half of the envelope allowing a view of the inner electrode structure.
A close-up of the top of the pinch and the base of the anode with grid and broken filament inbetween.
The balloon envelope is 50 mm in diameter and, excluding the B4 base pins, is 97 mm tall.
References: Adverts, boxes & 1003. Type B4 was first introduced in 1923.