Sonic the Hedgehog CD build v0.28A is a prototype of Sonic the Hedgehog CD for the Sega Mega-CD. It claims to have been built on 12th May, 1993.
Having been built just two days after the 510 prototype, this "512" version is extremely similar, with only minor changes across the board.
This prototype was discovered and acquired by drx of Hidden Palace in conjunction with The Cutting Room Floor, along with several other prototypes of Sonic CD and other Sonic games, and dumped as part of "Sonic Month 2019" from October to November 2019 . This prototype, along with the other Sonic CD prototypes, was released to the public on October 26, 2019.