This exhibit is an interesting mystery. The base cap is loose and was previously held in place by sticky tape. The identification is missing but there are two small patches of red paint that look like Mullard lettering as seen on 2D13C from the early 1930s. The top of the glass is re-entrant and reminds one of the EM4 envelope. The guess was that this was a voltage stabiliser and the B4 base limited the number of options. Under test the striking voltage was 125 Volts and the maintained voltage was close to 110 Volts. Only the 7475 came close to this set of values. The gas filling is neon.
The 7475 striking voltage is specified 140 volts with a burning of 100 Volts. Tube currents of 1.0 to 8.0 mA regulate to within 2.0 Volts. This exhibit has close regulation at those currents.
The top end of the envelope. The inside is opaque with the exception of a small circle at the centre.
The anode is a grey cylinder close to the glass and can be glimpsed in the top image. The cathode is not the usual inner small diameter cylinder but two small barrel like shapes either side of a central support. The opaque glass makes these structures difficult to see and very strong directional lighting was required in addition to the main studio flash units.
The wide glass tube envelope is 26 mm in diameter, and excluding the B4 base pins is 67 mm tall.