The 249B is a half wave mercury vapour rectifier designed for high voltage use. The datasheet indicates that it must be used vertically with the anode at the top. Like all mercury vapour rectifiers the filament must reach operating temperature before the anode supply is connected. In use the holder should also have pin one strapped to pin two and likewise pin three should be strapped to pin four.
The glass is partially opaque but with back lighting the main electrodes can be seen. Note the glass tube shielding the anode lead-out wire.
Looking into the cathode electrode. The filament is a wire mesh that is folded within the protective shield.
The balloon envelope is 66 mm in diameter, and excluding the UX4 base pins is 165 mm tall.
References: Datasheet. Type 249B was first introduced in 1936.