Just thought I would add that I've seen and taken pictures of original cars with both finishes. Not sure if its a supplier or tied to when the car was built ( San Jose 67 Disc brake Mustangs) but right now it appears to be a supplier thing IMHO. Often its impossible to determine the finish without touching and cleaning on an otherwise original but dirty undercarriage though in some cases like below the amount of rust (when compared to know painted and unpainted surfaces right next to it) helps in the determination
Suggest its a supplier thing since the two unrestored examples below were built one day apart at San Jose.
Finishes did match side to side which is interesting but seen much odder things through the years. Drivers and passenger sides both shown - one per car/example
I really wonder about the "supplier" theory. Photos do seem to supply evidence that suggest otherwise but I still wonder because my example that has not been driven in anything except Southern California desert area rain throughout it's pre-restored life, the brackets looked virtually just like the "unpainted" ones pictured in Jeff's pictures on the outside surfaces, evidence of a little minimal rusting, yet on the backside and in areas where grime had built up above the brackets, they are CLEARLY painted (dipped) black. THEN, add the fact that my frame rails in the same area are near perfectly preserved engine compartment black, needing only a light solvent cleaning, with most of the original underbody (salmon) color very nice and unscaved (further note: transfered some of the salmon underbody color stuck onto the black brackets, further indicating my example as unmolested over the years)
Again, at a glance, I can certainly see how somebody would believe them to be bare steel, while (using my example) in reality they were originally dipped.
To me, at least on what I am looking at, it is more of an "exposure to the elements" sort of thing. An unrestored in a good climate controlled storage would show all black, IMHO while another example that was driven daily would look like bare steel...Well, that's my theory at this point in time.
Richard
