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James, can you be more specific about 'what' was painted 'where' (fisher or assembly plant) and why you believe that (ie. evidences which support that?)...
There was some type of black primer applied before the body color was sprayed. I'm not saying more was not sprayed later, just saying at minimum one layer of black was on the firewall was there before body color paint.
When I find the color written in grease pencil on the firewall there is black under it, the color was not written after the car was painted.
The gray at the top of the photo is the factory gray primer.
In the above photo you can see how the peaks in the brush on sealer shielded the white from overspray. The dash semi gloss and the dash suede were clearly sprayed after the white for the wiper well as evidenced by the over spray patterns.
Here is an early one owner survivor LA car, 71 in grease pencil over the black.
At the top you can barely see some metallic silver
The fine specs are not dust they are silver over the black, I checked with a jewelers loupe.
Here is a gold black striped Z. with original paint, The gold mist is down over the firewall black.
Here is another unrestored LA car, look behind the engine, orange paint all over the firewall (over the black).