Nixie Tube IC Pin - What is it?
The IC in nixie tubes is defined as "Internal Connection". The pins are not supposed to be externally connected (equivalent of NC in other through-hole devices). 

The pin(s) provide mechanical stability, and in the case of circular pinouts such as the one listed in the question, prevent indexing ambiguity by ensuring that only one of the pin locations is "missing": 14 pin locations in a circle in this case, of which 13 have pins present.



IIRC (but cannot find a relevant datasheet at the moment) some nixie tubes designed for extremely low temperature use, have an internal heater coil wired from the IC pin to the Anode(s), and thus at Anode potential. This is to improve operational life of the tube, as operation below 10 degrees centigrade degrades the longevity of the tube. Please take this last paragraph as possibly faulty memory on my part, rather than as fact.