The HD24 is a directly heated double diode triode that would have been used in broadcast receivers, as detector, AGC detector and first audio amplifier.
This exhibit has a plain glass envelope whereas HD24 has external metalising.
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 HD24 has a lowest anode current rating of the series.
The reverse of the envelope has the Osram sticker.
Although obscured the anode can be made out as a bright box uner the top mica. The two diode anodes appear to form a slim box shape below the triode.
The classic envelope is 44 mm in diameter and, excluding the B5 base pins, is 106 mm tall.
References: Data-sheet, 1040 & 1043. Type HD24 was first introduced in 1939.