The Z803U thyratron is a cold cathode device. The device will conduct when a set voltage is reached on the control electrode and the ionisation within the device will then form an avalanche discharge.
The electrode tr is the trigger, and the electrode p is the priming electrode.
The thin glass tube envelope is 20 mm in diameter and, excluding the B9A base pins, is 33 mm tall.
Reference: Datasheet. Type Z803U was first introduced in 1955.