What is it? Gold thing on this master cyl

Some rumors about the cars say that some had anti-lock brakes. This would confirm the rumor and explain what they are. The odd thing is that I haven't been able to find anything like it on the web.
Trailbeast, Can you elaborate on who used to make those? "Passive anti-lock brake" doesn't come up with anything useful in google or bing...I'm assuming that you've seen them before...