The C2 barretter or current stabiliser was used with heater chains connected directly across the mains supply. The job of the barretter was to keep the heater current steady for fluctuations in the input voltage. In the case of the C2 the control range was 35 to 100 volts.
The filament is held on supports that branch out from a central glass rod.
The iron wire was coiled to allow a considerable length to be housed within the bulb.
Inside the wire can be seen to be held within a fold of the support.
The identification is etched into the dome.
The wide glass tube envelope is 38 mm in diameter, and excluding the Ct8 base pins is 113 mm tall.
Reference: Data-sheet. Type C2 was first introduced in 1934.