What about a search like "identifiers:isbn and not identifiers:amazon" ?
EDIT: Also, try to add a custom column, and click "isbn" on the new column form. When you've got Calibre restarted, sort by ISBN, enlarge the column, and see if there's one of 'em that doesn't look right.
Last edited by aleyx; 12-13-2013 at 02:01 PM.
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