The E2139 is an M-OV development sample and this exhibit has the serial number of 17. The box the exhibit was packed in says that the E2139 is a B7 based KT31 fitted with a two part IO base. The KT31 being a high slope output beam tetrode designed for series heater chain use. In the UK in the 1930s this would suggest use un AC/DC mains sets.
The Output load would be 5,500Ω, the cathode resistor 80Ω and the valve would produce 2.5 Watts.
The heater can be seen to be two sections of insulated coiled wire in series. The control grid is wound on copper supports.
The two part base that forms the basis of this experiment. The base shell is metal.
The classic envelope is 45 mm in diameter, and excluding the IO base pins is 110 mm tall.