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 Edison Screw base with pinch within. The envelope is filled with low pressure hydrogen.
The coiled iron wire is held within folds of the supports.
The balloon envelope is 56 mm in diameter, and excluding the base pins is 121 mm tall.