Over its lifetime, mu has experimented with different ways to deal with
development and release versions, trying to find the balance between getting new
functionality to users quickly, and minimizing the risk of introducing
regressions.
Currently (the 1.12.x series), all feature development happens on master,
with pre-releases having a version with a -pre<n> suffix, e.g.,
1.12.14-pre4. Every few months this is wrapped up in a release (on Github)
without that suffix, e.g. 1.12.14.
The version strings are parseble through Emacs’ version-to-list.