The HL2 is a dull emitter battery triode. It was designed as a detector in broadcast receivers. In 1934 it was redesigned in a smaller envelope and called the HL2K. The box gives the Type as the HL2M with the M blocked out. The exhibit itself has clear glass with no visible markings.
The front to back dimension is a maximum of 5 mm.
The valve has two top mica sheets, the upper one carries the filament tension spring.
A closer view of the ribbed anode and within the oxide coated filament and the close wound wire control grid.
The balloon envelope is 41 mm in diameter and, excluding the B4 base pins, is 85 mm tall.
References: Datasheet, 1040 & 1043 Type HL2 was first introduced in 1931.