OCLC number
Also called: WorldCat number, OCLC control number
The identifier WorldCat assigns to a bibliographic record, useful for locating a specific copy of a rare or old item.
OCLC numbers identify records in WorldCat, the shared catalogue behind most library discovery systems. For a modern book an ISBN is more useful, but for items with no ISBN — pre-1970 books, pamphlets, manuscripts, ephemera — the OCLC number may be the only stable handle.
Chicago allows it in citations of rare materials. Otherwise it belongs in your notes rather than your references.