http://www.geocities.com/lordazathoth/main.html

This guy mentions some Sonic CD beta with 3 levels completed. It might be SCD 510, as Borisz suggested. Already emailed the guy, waiting for a response. This thing would be nice to get our hands on, since we don't have a BIN/CUE. Also, seeing as there are so many SCD builds, it also might be some similar build, but not exact to 510.