The HL2K amplifier triode has a directly heated 2.0 Volt filament and an anode voltage of 150 Volts. This indicates a valve for battery HT operation and a 2.0 Volt accumulator for the filaments. This was almost certainly a valve for broadcast radio reception.
The HL2 was issued in 1931 and the HL2K in 1934. The K suffix was used for Catkin versions of existing valves and the rating was the same as the HL2. It is possible that this is a later replacement for the Catkin type, it certainly has a smaller envelope than the original 1931 balloon type.
The valve has a vertical box anode suggesting 1930s origin. The filament is a thin wire single inverted 'V'. The grid is hidden by the anode but the supporting rods can be seen protruding through the mica supports.
The shaped envelope and the Osram paper label.
The bright anode is ribbed for rigidity. The top mica has 'fingers' to fit tightly against the glass and side wings also designed to grip the glass. All devices to reduce microphony.
Here the secondary top mica can be seen. The filament tension spring ends in a hook to hold the inverted V filament.
The wide glass tube envelope is 22 mm in diameter and, excluding the B4 base pins, is 65 mm tall.
References: Data-sheet & 1043. Type HL2K was first introduced in 1934. See also1934 adverts.