[Marsupial]

From looking at the metal colored back of the backplate, it seems to have been sprayed (there is some overshoot on the otherwise metal side)

So, I'd think its some kind of metal paint that was damaged with years.
I could very well just refinish it. I don't quite know what type of paint I'd take. This has some texture to it. It seems to be a brownish bronze, or a bronzish brown...