Somewhere after R 2.1.0, I started noticing short phrases next to the R release number that is displayed on start up. This turns out to be a release nickname, an undocumented component added to the R.Version variable, accessible with
In response to an email asking if there is a system to the names, Peter Dalgaard states there is
"No system (except that they should be in season at release time)"
Thus, they are not alphabetical or otherwise ordinal -- there is apparently not a way to infer the order of releases from their nicknames.
There is, however, the appearance of a possible general theme:
As no one has uncovered any documentation, I'll tentatively accept my own answer.