If it looks like it was a wax pencil, then a chinagraph pencil suits the description. These were certainly around in the late 60s and early 70s as I used to have one in my old box of pencils.
It is a very similar thread to
http://forum.etypeuk.com/viewtopic.php?f=4&t=17811
Having said all that, anyone who claims to be a restorer and hands back to a client a car painted in 2pack paint, is probably guilty of far more than someone who scribes a few numbers which can be rubbed off. Just imagine if the Mona Lisa were given a nice big repro smile repainted in acrylic paint with "SNG Leonardo B" neatly stamped into the picture frame.
Perhaps conservation should become the new restoration.
kind regards
Marek