Very cool pics Skip.
I have an 05A Norwood build X33 LeMans Blue RS Z28 that I purchase back in 1995 and spent 4.5 years restoring. The car had just over 45K miles when I bought it. My original and dated coded fuel tank, and the top side of the interior heater core box, both had "63" markings with an orange grease marker. My firewall had numerous grease marker codes stains in the sheet metal, behind the blackout paint. Among the markings were "D8" for D80 spoilers, "X3" for X33, "10" which I assume was for N10 dual exhaust, and "2X" over by the large round hole for the heater core blower motor. When I get home from work this evening I will post a few photos.
As I read the Echoes of Norwood book on the history of the Norwood, OH assembly plant, it mentions how broad cast codes and sheets were used and communicated throughout the plant. A body build sequence number was used daily, starting with 001 each day and continuing thru the number of cars built that day. That number must have been "broadcast" thru the plant and the line workers numbered certain parts ready for the matching body number car coming down the line. Using a simple 3-digit number was much easier to see versus reading the print on the body or chassis broad cast sheets taped to the front window. I have seen the same 3-digiti number marked on the lower front valance panel and on inside surface of the spare tire in the trunk. However, I am not sure why fuel tanks and heater core boxes were marked as they are standard component used on any '69 Camaro. I understand the spare tire because rim/tire were designed for a particular car coming down the line.
The original color of the bottom side of my floorboards looked identical to yours...primary gray in appearance with black speckle overspray. My floorboards were NOT painted satin black as the current Legend judges mention they like to see. I have pictures from numerous survivor '69s with floorboards that have the appearance of 'light gray primer" color (from the baked phosphate body prep process) and only black with some overspray from the firewall blackout treatment and along the floorboards if the car received blackout on the lower door sills below the spear trim.
Love seeing your pictures!
Dave