For the built in McGraphOverlay select tool, this will be McGraphToolSelect and the ActiveClassName property will be an empty string. For the built-in McGraphOverlay creation tools, this will be an empty string and the ActiveClassName property will be the class name of the type of graph object being created (e.g., McGraphObjRect).
For a custom tool, this will be the version-independent ProgID of the tool (e.g., McOvrMgr.McZoomTool for the McZoomTool).