Even in 6th Edition Unix, devices were files in the filesystem as well. And in 6th Edition Unix, you could open and read a directory as a file; that was how you found out what its children were. There was no EISDIR. I believe this behavior persisted until at least SunOS 4.

Adding sockets and windows outside the filesystem is one of the things Rob is specifically complaining about here, and in fact complained about at the time as well.