I would guess that it's a remnant of some sort of glitch that happened. The "Preboot 2", if it is the same as "Preboot" when doing a file compare, usually means that the OS wanted to write the file to "Preboot" but found an existing directory of the same name and decided to just write it to "Preboot 2".
There is a separate volume in each APFS container which contains an OS called "Preboot" with subdirectories having that long alphanumeric string (C9F33840... in your case) which is the UID of the OS volume it corresponds with to. The Preboot directories that I have in my 10.13.5 install has quite a few more files than what you're showing. The date for the different files, I believe, is the date that it was installed - I could be wrong but to give a definitive answer would take quite a bit more work. I think if you were to look in the actual Preboot volume that is used, you'll find a date that is when you installed 10.13.4 and that will be later than what you have in your screenshot. Now if you don't have APFS, these files will still be on the disk (although I've done only a sampling of the files) so for example sound.efires is at
/usr/standalone/i386/EfiLoginUI/sound.efires on both APFS and non-APFS systems.
So looking at all that, you could probably delete the directories - but it's your computer, not mine. Certainly take precautions before doing that.
