Prototype 510 of Sonic CD, sometimes known as CD Sonic the Hedgehog after the odd word ordering on the title screen, is an early prototype of Sonic CD. Its compile date is May 10th, 1993. The prototype was discovered by a group of fans called Sega Extreme in Summer 2000. The original disc has since been sold, and is now lost.
Pictured here is a monitor showing Sonic's face can be found in the background at the very beginning of Quartz Quadrant Act 1 Good Future. This doesn't appear in the final. In both this build's Bad Future and the final version's Bad Future, there is an Eggman monitor in the same position. This Sonic monitor was restored in the 2011 rerelease of Sonic CD, appearing in Act 3 instead.