If the wiring harness on top of the battery box has just a Brown and a White/Green wire ... then, yes, it was for optional fog or driving lights, factory wired all the way to the instrument panel, and waiting for a switch to be installed!
Pelican has a colored, 'geographic-style' schematic diagram for your specific car here:
http://www.pelicanparts.com/911/911_Parts/911_electrical_diagrams.htm
It sounds like your car has been well taken care of over the decades!