While processor architectures are typically available for years, AMD and Intel typically offer improved steppings throughout a product's lifecycle. We looked at the history of the Athlon 64 X2 5000.
http://www.tomshardware.com/2008/03/20/amd_athlon_stepping_improvements/index.html
http://www.tomshardware.com/2008/03/20/amd_athlon_stepping_improvements/index.html