This is a Herman Miller Classic Aeron, labeled as #B5.  This chair is a 2015 model, it’s in excellent like new condition. #B5 has been thoroughly tested, and is mechanically 100 % functional. Forward & backward tilt, adjustable lumbar, fully adjustable arms, mesh is in excellent condition & has like new resistance. 
Upgrade to a posture fit back support for an additional 50, you can also purchase to an Aeron headrest for 50. Only with the purchase of a chair. 
Backrest will need to installed. Installation video on YouTube will be available, via shared link, upon  request.