This exhibit is a half wave mercury vapour rectifier. On test the helical filament glowed dull orange at 2.5 Volts but was a bright orange at 4.0 Volts. At 4.0 Volts the consumption was 4.3 Amps. 4.3 Amps seems high and so the filament may be designed for 3-3.5 Volts. The gap to the anode is large and this suggests a high voltage, although when in operation the voltage drop across an MV rectifier is small.
The filament ribbon looks much the same as the NU7.
A closer look at the filament.
The classic envelope is 39 mm in diameter, and excluding the B4 base pins is 103 mm tall.
