The GS12D is a bi-directional 12 way selector tube. The Ericsson trade name for this type of tube is Dekatron. The maximum count rate is 4,000 pulses per second.
The two flying leads from the base are the first and second guides connections. To move the glow from one cathode to the next involves pulsing the guides. The article above has the circuit details.
Above the pinch the many leads are held in and between ceramic discs.
The pinch broad face.
The central anode with cathodes and guide wires in the outer circle.
The base with central anode connection, locating spigot and colour coded guide wire connections.
The end window envelope is 32 mm in diameter, and excluding the base pins is 70 mm tall.