The Z303C is cold cathode bi-directional decade counter tube. It is neon filled and can operate at up to 4,000 pulses per second or 2 kHz sine wave. The display moves round the circle via the guide pulses and every 10 pulses an output pulse on c is produced.
The end window showing the many pins that guide the discharge and hold it in position.
The pinch and central ceramic disc construction. All the first guides are connected together within the ceramic disc. The same applies to the second guides and most of the cathodes.
Thanks to Mark Mallison for sending the images.
The end window envelope is 29 mm in diameter, and excluding the IO base pins is 87 mm tall.
References: Datasheet. Type Z303C was first introduced in 1952.