Bob, good to see you made your way here. We look forward to your input!
Regarding the engine bay, we are seeing body color under the black, so I would have to guess that it was done after the entire body was sprayed with body color. Maybe you are thinking of the priming process where the bottom of the front frame rails was coated? This coating could extend up quite a ways into the engine bay and outer front inner fenders. It was typically a different shade of primer as, like you mentioned, the belly of the unibody was sprayed with nozzles. Sometimes we see black underneath and red-oxide up under the front or all red-oxide with a different shade at the break of the firewall to the back. Then we sometimes see some slop colors, which as you mentioned, due to mixing all the drippings together.