The W21 is a variable-mu amplifier pentode (possibly made as a beam-tetrode as it is from M-OV) with a 2 Volt filament. It was built in two versions, this with a B7 base cap and a B4 based W21 version, both were released in 1937.
The filament is for accumulator use and the anode is designed for dry battery operation with a maximum anode voltage of 150 Volts. The operating conditions shown are for the near equivalent.
The Marconi branding.
The external metal paint is connected to the base via a wire that is held to the glass by this adhesive ring.
The balloon envelope is 42 mm in diameter, and excluding the B4 base pins is 102 mm tall.
References: Data-sheet, 1040 & 1043. Type W21 was first introduced in 1937. See also1937 adverts.