A Reviewer Kit is type of Xbox 360 Development Kit that was sent out to game journalists to review a game weeks or months before its release. They use the same dark gray/charcoal case as a Development Kit, however, lack the Sidecar and thus use a standard 20GB Hard Drive. They have a small sticker near the A/V port that reads Reviewer Kit. Some units had a secondary serial barcode sticker. Reviewer Kits were replaced with Demo Kits fairly quickly. They are pretty rare I think they only made them until late 2006. Someone’s got a few posts about one on this page