Kernighan and Pike challenge: how to put a slash in a filename?

Perhaps the answer is the same as part of the answer in this trick question:
How do you get down off an elephant? You don't. You get it from a goose.

From "The Practice of Programming" by Brian W. Kernighan and Rob Pike, Ch. 6, pg. 158:

When Steve Bourne was writing his Unix shell (which came to be known as the Bourne shell), he made a directory of 254 files with one-character names, one for each byte value except '\0' and slash, the two characters that cannot appear in Unix file names.

I've done this. This was on a UNIX system running on a PDP-11 sometime around 1980. I created a file called "WhatXNow?". I then used a binary file "editor" to edit the disk device and change the "X" to a "/" in the inode (with the file system unmounted).

The victim never figured out how to remove it.

Edit: whoops, Barmar is right, I failed to see the line in there about not patching the device. And yes, it was the directory I edited, not the inode. It's been a while :-)