The HD21 was always a Marconi valve, other M-OV brands supplied the HD22 that was biased at -3.0 Volts but essentially the same valve. The use is in battery superhet receivers. The triode as an AF voltage amplifier. One diode is the detector and the other the AGC rectifier.
The etched identification.
The Marconi brand applied as a paper label.
The right hand electrode system is the triode and has a single inverted V oxide coated filament. The left electrode tower also has the same anode and filament. The vertical struts that are grid supports in the triode are present and one acts as the second diode anode.
The electrode assemblies. The unconnected strut in the diode assembly is used to support the mica supports for the filament springs and lower down is a spacer that defines the separation of the two filament strands.
The classic envelope is 42 mm in diameter, and excluding the B5 base pins is 104 mm tall.
Reference: 1043. Type HD21 was first introduced in 1934.