The DRM_DRMHeader_ContentID attribute contains the contentID that was generated by the content owner.
This attribute is present with DRM Version 7 content only. It can be retrieved with IWMDRMReader::GetDRMProperty. To set the content ID on the file using IWMDRMWriter::SetDRMAttribute you must use the DRM_ContentID property.
Build date: 11/16/2013