> If this plate was
> made of iron, the iron balls would quickly rust to it.
Actually, more than that, even. Iron hitting iron can make sparks. Iron hitting brass does not. You could understand why in an environment like that, they'd want to keep sparks to a minimum ;-).
When working in explosive environments (my personal experience is in ethanol distillation towers), even today, most of the tools are brass or more lately, aluminum.