New Directions Paperbooks
Publisher: New Directions Press
ISBN format: 0-8112-xxxx-c (doesn't reflect series numbers)
Related Series: New Classics Series
In recent decades, the copyright page says “First published clothbound (ISBN: x-xxxx-xxxx-x) and as New Directions Paperbook zzz (ISBN: x-xxxx-xxxx-x) in yyyy.”
Starting at least as early as 1993, can look up [“new directions” “[title]”] on Amazon and see the copyright page. Can also view covers to verify if title is from special series.
New Directions to Publish Paperbound Editions, 168 PW 796-797 (Aug. 27, 1955) (announcing the October 1955 launch of the ND Paperbooks).
Recent catalogs indicating NDPs back to 2012: https://www.ndbooks.com/catalog/#/
Other Libraries, http://otherlibraries.com/ (not working as of 2020-06-18, but it had great scans)