The radiator is brass chambered copper
Other blocks are nickle plated copper.
I have an old ram block that has direct copper used to cool GPU ram, that block the copper has changed color completely (to reddish green) from copper color and has white fluff/dust around it.
The only reason I believe it is to do with copper is that inside radiator the brass is still very shiny, the nickel plates also pretty good and wipes clean, the copper plate is the only place where this is present, and the white fluff / dust does not come off easily, you have to really scrub it, the red/green tarnish does not come off at all unless you sand it.