Anodized black. The nicest, blackest one that I just rebuilt has the mirror hole filled with Bondo. I wonder what surprise lurks under that?
One lovely early black cap is cracked from a tip over, that's a shame. Every one has the fluid level line inside, no sandcast early ones. I think I'll order a couple rebuild kits and have them ready to go. It's cool how a little flushing and bleeding will keep them working.