The SPX is a pit of about 150 separate Market Makers. It is traded only at the CBOE. Unlike all the other stocks on the CBOE each market maker can't put up quotes on the screen. This can only be done by the Primary Market Makers. As of a couple of years ago, there were several of them and they rotated who could disseminate quotes. This is why the quotes are so wide in the index, as their are not multiple quoters.
Not sure who the current ones are, in the past it included Knight and Wolverine.
