The 5B21 is a gas filled half wave rectifier for low voltage high current applications such as charging lead-acid traction batteries. This type of rectifier is popularly known as Tungar for Tungsten and argon.
The filament is thoriated tungsten and the gas filling in these rectifiers is argon. The anode looks to be graphite.
The active arc is set in the centre of the spherical bulb.
The filament and anode.
The anode connection is via the re-entrant top of the glass envelope.
The Continental Electric Co name, the GES base and the pinch.
The Type designation and address - Geneva. In the USA.
The anode can be seen to screw onto the anode support rod.
The label from the base of the packing carton. The date is August 1956. The carton size is 160 mm square and 320 mm long. The valve was wrapped in a thick blanket of paper wadding.
The balloon envelope is 93 mm in diameter, and including the GES base is 215 mm tall.
References: Datasheet. Type 5B21 was first introduced in 1945.