As Mark noted below, the firewall markings on my 69Z (05A Norwood car) were found under the paint. Attached are a few pics.
The "X3" = RPO X33 for RS Z28
The "D8" = PPO D80 spoiler equipment
The "2X" = HD radiator, but mark has not yet been confirmed
The "10" = RPO N10 duel exhaust
What I have been told from a number of judges over the years, if you can photo document the marks under the paint, they will accept them on top of the firewall paint. However, since these numbers were typicallly not applied after the body/firewall was painted, I did not apply them to mine.