Nixie Tube IC Pin - What is it?

What is the purpose of the ic pin in a nixie tube? For example, the Z573M? Knowing how to drive one, this pin seems to have no purpose.

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).
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.