I just purchaced 25,000 mile two owner Z28 a few weeks ago. The second owner managed to slowly pull it apart over the last 35 years to make it a
"world of wheels car". I stripped the firewall of its gloss IMRON paint last week and found 3 places where the original crayon marks were.
I'm not sure if the original marks were under the black firewall paint or on the top but they are noticeable on the bare firewall now. One directly behind the distributor says " OK" . to the right of that ( passenger side ) it has " N10 " and i found out thats dual exhaust. and a little farther to the right is " D80".
Spoilers.. Pretty cool . Steve