Comment 4 for bug 1333031

OK, here are results of my small research:
"Ubuntu" is 'flavor' by default, if update-manager can't define it. The flavor is taken from installed meta-packages (ubuntu-desktop, kubuntu-desktop, etc) by dropping -desktop and capitalizing the word. To define elementary-desktop as meta-package, it should be added to MetaPkgs in /usr/share/ubuntu-release-upgrader/DistUpgrade.cfg ('ubuntu-release-upgrader-core' package).
"0.3" is just an output of "lsb_release -r -s".
Hope, this helps.