1 and 7 are not connected.
2 and 3 are signal ground
4 is shield (chassis) ground
5 and 6 are +15VDC

The pins are numbered clockwise, starting to the right of the notch.

The internal power converter can accept any voltage from 9V to 20V,
but I wouldn't recommend going below 12V.  You will need to supply
more current at lower voltages, and less at higher voltages.  It is
possible to run a IIc from a car battery (e.g. via the cigarette
lighter socket).