BZR nnn is a Bazaar revision number, Bazaar is a SCM supported by Launchpad. Since the revision number is unique, it was a handy way to tie a build to an exact source baseline.
So a previous release of KiCad was BZR 4022, nightly builds took on some other arbitrary but unique number.
However, now KiCad source is in git (but still on Launchpad), git has no concept of revision number, only a rather long hash value. Other ways to identify a source base line are by date, or a shortened git hash.
Since 4.0.0, I think all release builds are tagged 4.0.x, and dev builds tagged with a date and/or git hash.
tldr; BZR xxxx just means it was built when source was in Bazaar. No BZR number means it was built with source in git. To determine whether it was a release build or dev build, requires further analysis of the version string.