Did you read the link on the production process by John Hinckley?
IMO, the black was EVERYWHERE since that was the Phosphating/rust prevention step, PRIOR to primer of any kind, and prior to paint of course. First was the phosphating (rust prevention) - everywhere, Then the gray primer (heavier in some spots than others) and almost totally lacking in the rear portion of the driveshaft tunnel, heavy where the gas tank was going, and heavy gray primer in the transmission portion of the tunnel (and probably down from the firewall - prior to the body paint on the body and firewal... everything else on the floor was 'overspray from priming/painting the body, and the 'last overspray' under the floor would have been the black from the rocker blackout for those cars that got it...
I wasn't there when they painted these cars, but based on John's description of the overall process- and there's no one better to describe how the plant functioned on these cars! ... and what I observed when cleaning under my car, that is what I think they did. Of course, there could also be variations from plant to plant (Norwood and LA), and possibly variations during the year and from car to car?
The overall 'thing' I learned was that there was NO attention for 'pretty' detail UNDER the car. All the paint/coatings/overspray/etc UNDER the car came from 1) GM/Fisher's attempt to rustproof and seal the body shell, And 'residue' from the body paint process... Buyers didn't inspect UNDER the car prior to purchase...
