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 shell has a central groove and this base shell is typically found on M-OV valves. The Bakelite on the base infill has the BTH logo. This base shell type was in use around 1923-4.
The box is 90 mm square and 124 mm long. Inside of the box was the original 1924 data sheet as can be seen below.
The envelope itself is without any markings.
The balloon envelope is 48 mm in diameter and, excluding the B4 base pins, is 98 mm tall.
References: Adverts, boxes & 1003. Type B4 was first introduced in 1923.