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(discuss.tchncs.de)
Post funny things about programming here! (Or just rant about your favourite programming language.)
This is in no way unique to IBM manuals.
The phrase is present in loads of books and in some cases has been for quite a few years.
https://meh.com/forum/topics/nothing-is-lacking-the-long-history-of-intentionally-blank-pages
https://www.google.com/search?q=%22this+page+intentionally+left+blank&udm=36