The HD23 is a directly heated double diode triode used in broadcast receivers, as detector, AGC detector and first audio amplifier.
The HD24 has a lower filament consumption than the HD23 which itself was designed to lower the filament consumption and thus replace the HD21 and/or HD22. The HD23 and HD24 have a higher ra than the earlier types.
The wire that can be seen on the left originally connected to the metalising paint. All of the zinc paint has flaked off of this valve. The triode is at the top with the two diode anodes appearing to form a box.
The single inverted V filament can be seen to run from bottom to top through both the diodes and the triode section. A single spring maintains tension.
The classic envelope is 40 mm in diameter and, excluding the B5 base pins, is 98 mm tall.
References: Data-sheet, 1040 & 1043. Type HD23 was first introduced in 1938.