The VGT128 argon filled thyratron is from the 1930s and was used in timebases etc. This valve was made for the Government controlled General Post Office that ran the telephone system and various government communications.
The worlds first computer, Colossus, used these thyratrons as pulse generators. The operating speed of Colossus was 5,000 cycles per second.
The commercial GT1C Type designation contained within the M-OV ellipse.
The shielded anode and trigger electrodes at the base of the image with the heater and cathode at the top.
The GPO and VT189 are etched into the envelope.
The classic envelope is 44 mm in diameter, and excluding the B5 base pins, is 101 mm tall.
References: Datasheet, 1040 & 1043. Type VGT128 was first introduced in 1937.