The 301 is a Barretter or current stabiliser for use in series with 300 mA valves in a series heater chain. It protects the valves from inrush current when the heaters are cold and thus of low resistance and maintains the correct current for fluctuations in mains voltages.
The iron wire is maintained in a hydrogen atmosphere and the single non-coiled strand is festooned between the supports held in a central glass rod.
A closer view of one support and the wire looped around it. The ends of the wire are held in folds of the support arms.
The balloon envelope is 64 mm in diameter, and excluding the base pins is 133 mm tall.