The documentation seems a little vague on what "tag" and "version number" formats actually are (and it's definitely changed at least between 1.9 to 3.1.0 which is what I've ended up testing by accident!).

As of 3.1.0 at least it appears that:

• {tag} can have a leading "v", which will get stripped in the version number setuptools_scm generates
• the {scm letter} is "h" for mercurial and "g" for git (makes perfect sense, but having that documented might have alerted me to the fact something was odd when I got "n" instead - I was on an old version).

The docs also say:

For git projects, the version relies on git describe, so you will see an additional g prepended to the {revision hash}.

but AFAIK I can tell this is actually just the "{scm letter}" itself, i.e. it's not actually "additional"?

Hopefully this is helpful - I stumbled on setuptools_scm and it looks really really useful so thanks.