This refers to the couple of 280 I've had apart. All those parts mentioned are black, the servo pump mounting bracket, fluid reservoir mount, all black. The band holding the fluid reservoir to the mounting bracket is not aluminum(at least not on any of the 280s I've seen.
It's a Norma clamp, size 94mm. It could of course be something different on a 250 if that's what you are looking at.
That's not to say that differences in use of material on a 250 and 280 does not exist, they do. I just had a discussion with a member on the german 113 board about the metal used in optional roof rack moldings found on some pagoda hardtops.
I insisted on anodized aluminum for my 1970 280, and he claimed they were brass on his late 250, since he had successfully rechromed them. Turned out they changed from brass to aluminum sometime in 1969 for those moldings.