They are hard to read because they were placed there during body fab, on raw steel. One of the last operations was firewall black out, when they were covered with paint. It wasn't necessary to write TEAL on the firewall after the car was painted. Over time the paint flakes off and they become a bit more visible. The 'firewall graffiti' often seen such as M-21, L78, z-28 is nonsense; wasn't visible after firewall blackout.
They are easy to read if the firewall is media blasted because grease pencil stains the metal.
The only marking occasionally seen on the firewall blackout is the assembly sequence number.
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