I noticed one more interesting thing in Business Central CU06: The BuildBranch information format in Codeunit 9015 "Application System Constants" has been changed.
The old format was (CU05) "D365F024", where the last number part was a running number. The new way starting from CU06 is "NAV13x".
What does this mean? Your guess is as good as mine!
I actually noticed the same (!). My guess was that these are just internal codes used during builds. If you have access to the insider builds, then they look quite different depending on (my guess) which branch it comes from.