The reason one might see 'brassing' in worn areas is because there is a bronze colored layer of paint between the gray plastic and the outer layer of black paint. It's plain to see looking at the underside of this particular top plate. I think that's pretty clever, although, I would have preferred metal over plastic.
Brad, since your Nikon FA top plate has a dent in it, perhaps some earlier models were metal. I don't know.
I can send photos if anyone is interested in seeing both sides of a Nikon FA top plate. I'm usually not this technically 'vocal', but I really love using FAs (as far as 35mm goes). -Marc