1965 Dodge Dart Charger

Terne plating used to be a low-melt lead/zinc dip, back before seat belts were necessary, back before lead and cigarettes were bad for us. Now it's not necessarily tin/lead but might be tin/zinc. Article here gives the basics but can't necessarily be relied on for precise accuracy; there's no law against lead/zinc terne and it's very likely still in use. There's also Ni-terne, which is when the bare steel is nickel plated before the terne dip -- Ni-terne has long been common in fuel tank manufacture, and while I can't come close to swearing to it, it would be a fairly safe bet that OE tanks were Ni-terned.

As for what it looks like, whether Ni- or not, it doesn't have that granular/crystalline appearance like hot-dip galvanize. Terne is pretty smooth and uniform in appearance; it starts out a semi-dull silvery tone then gradually ages to a darker grey.

As for the J-bolts, I would be surprised if they were bare steel. Probably cadmium plated or chromated, originally.

(Thanks for the props, Leanna! I think that black crud you saw might have been some kinda graphite compound or other process-related gunkum; it doesn't sound like a deliberate coating which would've been more uniform and not streaky.)