The GL3C23 is a thyratron with both inert gas and mercury vapour filling. The design is to allow for higher anode voltages than inert gas thyratrons alone can provide. The deionisation time of 1 ms indicates a low repetition rate of up to 400 cycles per second..
The pinch and lower components the filament supports are substantial as the current is seven Amps. Tiny specs of mercury can be seen on the glass.
The perforated cylinder is supported by a collar clamped around the pinch stem. The top of the cylinder is also perforated and it is connected through the pinch stem to the base and is thus the control element. The anode is a disc connected and supported by the top cap.
The classic envelope is 51 mm in diameter and, excluding the UX4 base pins, is 135 mm tall.
Reference: Datasheet. Type 3C23 was first introduced in 1945.