The HL2K amplifier triode has a directly heated 2.0 Volt filament and a maximum anode voltage of 150 Volts. This is a valve for battery HT operation and a 2.0 Volt accumulator for the filament.
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' with tension being maintained by the spring at the top. The grid is a zig-zag wind in the notched rods.
The secondary mica supporting the filament tension spring with the hook holding the top of the filament. The dome carries the etched Type designation.
The wide glass tube envelope is 25 mm in diameter and, excluding the B4 base pins, is 64 mm tall.
References: Datasheet & 1043. Type HL2K was first introduced in 1934.