o This is probably the cause of the confusion we've had trying to work out what pre-releases are actually building.
  * Why do some have a version key and some don't? How is it automatically added?

Nothing happens automatically for that.
Whatever a maintainer puts into that file will be used: a branch name, a tag name or nothing which implies the default branch.

Ok, crystal now. I was wondering if source.yaml shouldn't be touched (I can't remember doing any pull requests to source.yaml, just to release.yaml, so something must be auto-generating the initial footprint there).