Updated models with AMD graphics options expected in early 2017.
Apple Seeds Mac OS X 10.5.5 (9F5)
Apple instructs developers to focus on testing a number of areas including Time Machine, AirPort, Networking, Spotlight, and many others. Apple details over 83 other bug fixes and issues addressed including a few potential kernel panic scenarios.
At this early stage, there appear to be some major issues as Apple warns developers not to install this seed on the MacBook Air, MacBook, Mac Mini, iMac (with Intel integrated graphics).