If I'm not mistaken this is a reproduction from the original writer's guide: http://www.cloudster.com/Sets&Vehicl...se/eplan01.JPG
Compare it to the quoted / illustrated text from the original Star Trek Concordance
and you'll notice that the "Constitution Class" part has deliberately been added to make it appear
official (I consider this to be fraudulent behaviour).
One of the issues Gene Roddenberry felt very strong about was that "U.S.S." was an acronym for "United Space Ship" (or "United Star Ship") to make the show and the future palatable to international audiences. Of course, a "Constitution Class" carries strong allusions to American history and tradition (because of the US frigate of the same name and the United States Constitution), but I was always under the impression that Gene Roddenberry tried to keep such allusions to a minimum and "Starship Class" is an obvious proof for that, IMHO.