what is this bit on my heater for?

[disasterbus]

Never paid much attention to it as this is my first Chevy truck, but there is a round bit like a flag pole holder on top of my heater.
Last week Martin (mjh1965) and Rian (grizz1963) came over and gave the truck a once over. None of us know what that bit is or what it does.



My truck is a '64, their's are a '65 and a '66, but neither of their trucks has that bit on top of the heater box.

What on earth is it?