All 4 backing plates on my 69 Barracuda were phosphated. Yours might be too. Phosphating (or Parkerizing) is the gun metal gray color of military weapons, most Mopar bolts, hammers, crowbars, etc. These are all black, but you can't lift up any paint off them.

You can find good examples of phosphated fasteners if you look at your steering column bracket. That's the appearance - it's not a paint. The indented head machine screw with captive washer and "23" on the head in the photo is phosphated.

You can do phosphating at home yourself with an electric skillet, camp stove, or similar. You just need to get a chemical solution up to 200 deg in a stainless pan and dip, after totally all rust and residue is removed from the part.

Google Parkerizing or phosphating to see how the gunsmiths do it. Once you get the supplies, it's easy. The hardest part for me has been to keep my solution at 200 deg and not let it creep up to 212 deg boiling.