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For the most part Houghton will be adding an 007 field to holdings records for "service copies", i.e. positive microfilm for the use of readers. A typical positive service copy would be coded: |a $$a hd|afb---baca
Master negatives require slightly different coding, e.g.: |a for example: $$a hd|bmb---baca
(but see the MARC field documentation for precise codes). The third byte contains either a blank or a fill character.