http://www.softwarepreservation.org/projects/c_plus_plus/ind... leads me to http://www.softwarepreservation.org/projects/c_plus_plus/cfr..., which shows that the first external C++ release didn't have // comments.
The first commercial release had, though (http://www.softwarepreservation.org/projects/c_plus_plus/cfr...)
I was only thinking of the standardized ones, but you're correct, of course!