More help hopefully
Some examples of the lower firewall to floor color transision
As mentioned the painter for the engine compartment black could choose to be lazy and not put allot of effort in seeing that this surface was well coated. Below is an extermine example IMHO where the jets did a poor initial spray and the worker made little or no effort to cover this area
In other examples the shape of the firewall, the angle and position of the gun as well as making a single pass could produce shadows and in turn bare or thin spots that later rusted. Not unusual to find nice long runs of paint from the engine compartment black application
As mentioned trying to see the engine compartment black ends and the darker batch color begins can be difficult
Picture of a fully covered, with batch, torque box. After looking at allot of these cars it appears that the outer jets were staggered forward from the center jets which appear to start more rearward. Possibly and extra row on those outer corners to produce what I'm finding
More evidence of those forward outer jets as we find floor color further forward from the center point paint start. In this example the batch is forward of where a power steering ram bracket would be attached if so equipped. This was covered by the forward frame color applied by hand