Bob, the most common places to find the inspection marks are under the hood, the actual hood. The firewalls also usually have the ink pencil marks around or under the wiper transmissions bolted to the firewall. It is also common to see a left and right mark, always different, signaling it was being inspected from both sides as it traveled down the line- mostly only found on step-downs. The ink pencil marks on the firewall must have defined the build, as they are always factory painted over. It is also common to find final markings, not painted over, obviously done as a final inspection. Most of the marks I have seen over the years must have been the inspectors employee number or a distinctive letter/letters that signified completion and probably inspection of serial numbers matching the body??. Other markings for earlier cars were different yet in some ways.  '47 and earlier cars had markings on the back of the actual dash panels, like if was a truck, common to see, "cab Only" on the back of that panel. These are all only guesses, as I have never seen a written description of these markings. Maybe someone on here has some info that could enlighten all of us?. Good luck