The Supplemental index was started in August, 2003. It kicks in when Google runs out of results to show from the main index.
The URL-only index isn't really an index, but just a listing of URLs. Your search term has to hit on the domain, or directory, or filename somehow. The page itself isn't indexed, but the words in the URL must be indexed for fast access. That's the only sense in which it can be called an "index." A URL-only listing is a more accurate term, so as not to imply that the page itself is indexed.
Now, are you suggesting, renee, that the sandbox is yet another index?
If so, my guess is that they're shelving stuff temporarily because they anticipate rolling out a new main index with an expanded docID that has more than 32 bits, Real Soon Now. The Supplemental and URL-only listing is bad enough, but if they added yet another index to prop up their shaky operation, I cannot see how they can justify this unless the Big Fix is right around the corner.