This tetrode thyratron is built on a conventional glass pinch. The same valve from STC uses a glass disc base, the forerunner of the modern all glass construction.
The control ratio is high at 350 and the voltage drop across the tube when conducting is 9 Volts.
Below the lower mica can be seen, left to right, cathode with heater within, control electrode and encased in glass is another connection. The box is the anode..
The box seam.
Close-up. The cathode is on the right.
The classic envelope is 38 mm in diameter and, excluding the IO base pins, is 88 mm tall.
Reference: Data-sheet. Type 20A2 was first introduced in 1957. See also1957 adverts.