The C9 is a barretter, that is a series resistance to stabilise current flow. The barretter characteristic comes from the use of iron wire in a hydrogen atmosphere. The attached data sheet has a good explanation of the barretter and its function. The C9 was for 200 mA heater chains and the current remains constant for voltages of 60 to 100 volts.
The introduction of AC mains in the UK was a gradual affair with some areas opting for DC supplies and remote areas having no mains electricity. DC mains supplies could not be changed to other voltages with transformers and so for radio receivers the 'Universal' sets were introduced. The HT came straight of the mains and the valve heaters were series connected with a dropping resistor also across the mains. Mains voltages can fluctuate and thus so will heater current and cathode temperature. The barretter would keep the current constant over a small range of changing voltages. When DC mains were replaced with AC the need for universal valves reduced. However the series heater chain came back into fashion with the post war television receiver.
The coiled wire is supported around the central glass core and terminates in folds of the pinch connections.
The top of the glass support and the outer envelope. The envelope carries the Tungsram name and the Type designation.
The wide glass tube envelope is 37 mm in diameter, and excluding the Ct8 base pins is 129 mm tall.