The 2G is a Barretter or current stabiliser. Iron wire in a hydrogen atmosphere forms the active element. When inserted in series with a 100 mA heater chain it can maintain the heater current sensibly constant for variations in the mains voltage between 5 and 40 Volts.
Baretters cannot be connected directly across a voltage source.
Inside the envelope are two separate wire loops. One loop connects to pins 1 and 4 and the other to pins 2 and 3.
The wire loop is made from coiled wire. Each loop has a cold resistance of 30 Ohms.
The wide glass tube envelope is 26 mm in diameter, and excluding the base pins is 83 mm tall.