From Wikipedia's code page article:
Originally, the code page numbers referred to the page numbers in the IBM standard character set manual
Is any version of this publication available? I'd be content even with a photo of the cover.
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The document everything seems to refer back to is IBM's "Character Data Representation Architecture Reference and Registry" (SC09-2190-00).
There is a copy here, in IBM's BookManager format: https://www-01.ibm.com/servers/resourcelink/svc00100.nsf/pages/zosv1r13-pdf-download?OpenDocument (search for SC09-2190, about three-quarters of the way down a long web page).
This site claims to be a BookManager reader that runs in on Windows 95 and similar Windows versions: https://www.ibm.com/support/pages/ibm-library-reader-windows.
The FTP download link works, but I don't have any way to install it on such an old OS - so apologies if this answer leads nowhere.
It may be a warning flag that most documents on that long IBM list are available as PDFs, but not this one. I guess it may be unreadable unless all the code pages are actually installed somewhere the document reader can find them.