The HL210 is a general purpose battery triode with a two volt filament. This example is unusual in that the dome is deformed. Like HL210 The only markings on the valve are the Type designation HL210 near the base.
The deformed glass dome. At no point does the glass touch the inner electrodes and so it is not an exercise in supporting the valve against vibration.
The glass is clear for half the circumference and so the fine wire grid and oxide coated filament are easy to see.
A closer view.
The balloon envelope is 45 mm in diameter and, excluding the B4 base pins, is 87 mm tall.
References: Data-sheet. Type HL210 was first introduced in 1929. See also1929 adverts.
