This BTH valve seems to be labelled B4 but the B4 has a 6 Volt filament, as this is a later product than the 1923-4 B4 it is possibly a replacement type.
The filament voltage is marked as 2.8 Volts, suggesting dry battery operation. The internal envelope silvering obscures the electrode assembly.
The balloon envelope diameter is 35 mm and, excluding the B4 base pins, is 85 mm tall.
References: Adverts, boxes & 1003. Type B4 was first introduced in 1923.
Absolute Maximum Operating Conditions¶
Updated May 21, 2015. shape:balloon construction:pinch type:triode age:1920.1930 base:b4 heater:6.0v pins:4 pin:1.a pin:2.g1 pin:3.f pin:4.f