[JdeBP]

My guess is that it was undocumented until Release 3 or 4, so the book authors simply never found out about it. Andrew Tanenbaum knew about it at the same time as the first Hunter and Fielding book, but likely this was word-of-mouth stuff.

* https://groups.google.com/g/comp.unix.wizards/c/bbCYhh7_r-8/...

Enjoy the obvious question asked about all this, way back in November 1981:

* https://groups.google.com/g/fa.unix-wizards/c/h5mFGIV91Io/m/...

That's the Dave Yost, still at RAND at the time, who went on to make the GRAND Editor.

* https://news.ycombinator.com/item?id=43734536