OK, I'm looking at the parts book, and my car. There are eight fastener's holding that piece in. The lower forward one goes through the front of the firewall into a weld nut. The three rearmost bolts go into weld nuts in the in the master cyl. housing, and then there are four studs that are welded to the piece you are taking out that go up though the master housing and are held up top with nuts. I do not see any other fasteners.
Maybe one of those studs broke off, and the previous owner replaced it with a nut and bolt.
Edit: Give me call if you're still having trouble.