Z701U is a cold cathode trigger tube with two trigger electrodes. It features a very low noise level and a speech frequency impedance of less than 1 kΩ.
Potential uses would be in calculators and probably telephone exchanges where switching of speech would be the role.
There is some suggestion that the trigger voltage is low due to the presence of tritium gas (half life 12 years).
Side view, there looks to be two plate electrodes and a rod.
The rod is placed centrally between the two plates.
The thin glass tube envelope is 9.5 mm in diameter and, excluding the B8D/F base wires, is 42 mm tall.
References: Data-sheet. Type Z701U was first introduced in 1960. See also1960 adverts.
