The Mullard AN1 thyratron is better known by the GEC Type designation of GT1C. It was used in timebases for Radar. Thyratron was a trade name for a gas filled triode. In the case of the AN1 the low pressure gas was Argon.
At a defined grid voltage, the valve will rapidly switch to full conduction. The valve is turned off by reducing the anode voltage. Thyratrons are gas filled and can be considered as triggered gas discharges. The precision of construction gives the defined and reproducible operating characteristics. This valve not only has a high peak current but is designed for operation at up to 8,000 pulses per second.
The worlds first computer, Colossus, used this type of thyratron as pulse generators. The operating speed of Colossus was 5,000 machine cycles per second.
It is hard to read but the reverse is also marked as CV1128.
The anode is a rectangular box.
The wide glass tube envelope is 29 mm in diameter and, excluding the B5 base pins, is 93 mm tall.
References: Datasheet, 1040 & 1043. Type AN1 was first introduced in 1937.