Later factory cars did incorporate rear window defrost. I do not know when this became available.
of course, in the 48 years since your car left Italy, without original records it becomes very difficult to know what may or may not have been installed at the factory.
I would be surprised if there were many street driven Panteras built without air-conditioning, so I imagine the location of the defrost control switch replaces the OEM air conditioning switch.
to me, this points to a previous owner having installed the window, and choosing an existing hole for the control switch
Larry