Flash MX 2004.
item.linkageClassName
Property; a string that specifies the ActionScript 2.0 class that will be associated with the symbol. (To specify this value for an ActionScript 3.0 class, use item.linkageBaseClass ) For this property to be defined, the item.linkageExportForAS and/or item.linkageExportForRS properties must be set to true, and the item.linkageImportForRS property must be set to false.
The following example specifies that the ActionScript 2.0 class name associated with the first item in the library ismyClass:
fl.getDocumentDOM().library.items[0].linkageClassName = "myClass";