Of pewter, with probably at least 88% tin with antimony, cadmium, a pinch of zinc and maybe a few other ingredients.
An alchemist's nightmare I would think.
They are AB (Antiqued Brass) finished or AP (Antiqued Pewter) finished.
I wish the finish was always uniform. It isn't.
Sometimes the AP seems more a shiny nickel and the AB a smudgy dinge.

Pewter is not a sturdy metal. No one is building hubcaps out of it.