This exhibit has no visible markings. A previous owner had determined that the filament was 4.0 Volts and that the μ was 0.5. That probably should have been the gm. Sadly the filament is now open circuit and so no testing has been possible.
The envelope shape looks like an early Mullard design.
The balloon envelope is 54 mm in diameter and, excluding the B4 base pins, is 91 mm tall.
References: Observation and previous collector.
Updated October 01, 2019. shape:balloon construction:pinch age:1930.1940 base:b4 pins:4 pin:1.a pin:2.g1 pin:3.f pin:4.f