The Tungsram HR210 is a battery triode and is equivalent to the Mullard PM1HL etc. These very popular valves were used as detectors. This exhibit has a plain clear glass envelope, whereas HR210 has metallic paint external screening.
Another HR210 in the museum has a balloon envelope.
Side view of the bright anode.
The dome carries the Tungsram name and the Type designation. It also declares that this was made in England.
Looking into the top of the glass, the oxide coated filament can be seen looped over the tension spring.
The classic envelope is 39 mm in diameter, and excluding the B4 base pins is 83 mm tall.
References: Datasheet & 1043. Type HR210 was first introduced in 1933.