The XB1 is a barretter or current stabilising device. Normally barretters have iron wire as the active element in a hydrogen atmosphere. The XB1 was designed to stabilise a current of 300 mA in heater chains. In use the voltage drop was designed to be 9 to 16 volts and the current to be stabilised at that voltage drop.
The iron wire is draped over two supports.
A closer view of the coiled wire
The thin glass tube envelope is 18 mm in diameter, and excluding the B7G base pins, is 46 mm tall.