The Herman Miller Aeron has been available on the market since 1994 and since then there have been a few subtle changes to the parts since then; the chairs have been referred to as the mark I (production runs from 1994 - ~ Aug 2004) and mark II (production after Aug 2004). All of this matters to avoiding some subtle pitfalls if you are looking at a second hand Aeron chair from various office clearance companies or ebay.
The chair has many options for configuring the type mesh and lumbar support but the main configuration that can't change involve the support for full front and rear tilts (fully loaded) or just rear tilt. The chair can also be configured to have no arms, fixed position arms or up/down adjustable arms: the arms can also be fixed or can be pivoted in 3 positions (in/out/middle).
Below are some of my experiences from trying to find my own Aeron chair, and subsequently fully refurbishing a 1999 chair, which may be useful to anyone trying to determine if an Aeron bargain is worth the risk.