*15.5(1)SY is the current active extended maitenance release.
Engineering Special Releases
In certain cases, critical bugs are made available on Engineering Special (ES) builds. These builds are not available on CCO (cisco.com). If you are running into any issues specific to your environment which is not already fixed in existing cisco.com releases, please contact Cisco Customer Experience (CX) to explore the possibility of an Engineering Special (ES) code. Engineering Special builds are meant to be deployed for a limited time frame and customers are advised to move to the next maintenance rebuild as soon as it is available on cisco.com. Engineering special builds are fully supported by the Cisco Customer Experience (CX) and the Business Unit (BU).