Maybe that was a typo for 'mkdir as a system call'? (I recall having to do a
fork() to execute 'mkdir', back when.) But 4.2 had mkdir().
I believe, it was 4.2 (or 4.1c) according to David Tilbrook's paper on rename. The site
qef.com seems to have disappeard, but the wayback machine has copies[1]
Timing being everything, I happily have a copy of it all, as I used wget to pull it into a git repo.
It's doubtless not in "native" form as he used one of his little languages (that were part of qef) to generate it. But HTML isn't too bad for this.