The GN-4D is a cold cathode numerical indicator tube.
The construction is of a common electrode normally the anode, and a series of shaped cathodes. Each cathode being in the shape of a number from 0 to 9. This were stacked on support rods and each cathode was brought out to its own pin. Energised by around 180 volts a open collector based TTL chip would interface between the digital circuitry and the display. As each cathode was connected to ground the glow of the neon gas filling would form found the cathode and through the end window the shape of the numeral would be visible.
Seen end on the mesh is clear. The numerals seem to be jumble of metal when not in operation but once energised and the discharge is seen through a red or orange filter the numerals are clear and distinct.
The insulated cathode supports are visible. Also one can see the wires emerging from the cathodes, between the insulators, and going to the base pins. In the centre left one can see one pin shrouded by a ceramic insulator.
This shows the base and the arrangement of the 13 pins.
The end window envelope is 27 mm in diameter, and excluding the B13B base pins is 26 mm tall.