Power Steering Blues

[phat_toaster]

So, as promised, I jumped into the struts and replaced all four corners today, but that's a topic for a different thread. I found what I can only surmise is the the filler for the power steering fluid, but it's a tube on the driver's side that routes directly into the firewall, and the cap says "Use only DOT 3 brake fluid." And there was also some strange black plastic disc inside the nozzle, beneath the cap. Almost like a perfect plastic stone. Is that the filler for the power steering fluid? If not, what the heck IS that thing?
P.S. -- After about 50 miles of not working, the power steering pump seems to have come back from its leave of absence, at least temporarily.