Here are several assorted pictures of firewall stampings from original cars. Everyone seems to have their own theory as what they relate to, and I'm sure someone is correct. I've heard that they are simply inspection marks stamped on the cars as they progressed down the assembly line. The colors and initials of the marks may represent somthing, but I'm unaware of that at this time.
Paul Panczuk took matters into his own hands during his restoration and made his own stamps from what he had seen from original cars. He went to a local hobby/craft store and had them make the rubber stamps, apparently there are many places available that will make rubber stamps. 1st picture is the stamps and the 2nd picture is my car with the stamps.
Found the following from the Yahoo Hurst/Olds group:
Dave Bunch - former Lansing Assembly Plant exc. employee in Product Reliability and Engineering. 1968-73.
These stamps were applied right after "body drop". This is when the body is mated to the chassis at the end of the "chassis build line" at the assembly plant. In the body drop pit are usually 4 workers who install the rubber body mounts just prior to full body drop. Then after the body is positioned properly over each mount, then the body is completely dropped on to the chassis and all body hardware is installed and tighened to a specfic torque. Then they indicate that this is completed, the "Body Drop Station Inspector(s)" stamp the firewall with their stamp letter(s). This stamp letter(s) is assigned to a specfic inspector, indicting the full name of the inspector is. This is both a quality control and safety issue. If the body is not aligned properly the rest of the front end assembly will not be accomplised according to the assembly manual. As a safety issue, this is the fact that the front end/ body will or may not meet design spec in a crash