For the sake of anybody else interested, I was able to play with a new Kobo device (a Libra 2) purchased recently, which included 52 previews when the KoboTouch[Extended] driver's “Show previews” option is enabled. They range from about 40 kB to 7.5 MB and include titles in a variety of languages. These files are located under the .kobo/kepub/ directory relative to the root of the device.
I have confirmed using the cmp utility from GNU diffutils that these files are byte-for-byte identical to those shared earlier at this link:
https://www92.zippyshare.com/v/HzV4fwHw/file.html
The device has only been started in sideload mode so far (by altering the configuration file without going through the setup process). While the books do not appear in My Books, the books' authors are listed in the device's Authors view, which is a little surprising. Touching any given author's name takes you to a screen showing zero books by that author, which is both surprising and frustrating. I have not experimented with what happens when the device completes setup in the normal way.
Mystery solved – the 52 books were previews.