For Harvard practice, see:
http://isites.harvard.edu/fs/docs/icb.topic228994.files/Harvard_Local_Subject_Headings.htm
69X fields are frequently found on keyed records for Houghton books and Widener transfers. When possible, these should be deleted and replaced with standard access points when the records are upgraded; but if they were added by a library other than Houghton or Widener, do not delete.