The HL2 is a dull emitter battery triode. It was designed as a detector in broadcast receivers, in the early 1930s. This exhibit looks to be a M-OV product. It is without its base and we can see how the envelope and stem are joined. HL2 is also of M-OV origin.
The box anode side on. The glass bead insulator holds the filament tension spring.
The filament tension spring at the top holds the point of the single inverted V filament. The gauze on the right is the getter holder.
The evacuation tube connected to the stem. The filament wires are insulated.
The shaped envelope meets the stem and the fusion roll of glass that forms the join.
The filament tension spring and with enhancement the HL2 can be seen on the envelope dome. The insulator is also fixed to the grid supports above the top mica.
The reflection off the far wall shows the holes in the gauze where the getter has been fired.
The balloon envelope is 40 mm in diameter and, excluding the B4 base pins, is 82 mm tall.
References: Datasheet & 1040 Type HL2 was first introduced in 1931.