When
http://classify.oclc.org/classify2 can't find an OCLC number from an ISBN,
https://www.worldcat.org/isbn/ should be used instead. It immediately does a 303 redirect to a URL containing the OCLC #.
For example:
http://classify.oclc.org/classify2/C...=9781505115932
doesn't return any results, but
https://www.worldcat.org/isbn/9781505115932
immediately redirects to
https://www.worldcat.org/title/rules...clc/1145597595
The OCLC # is at the end: 1145597595