1) This ship named USS Enterprise is based on an old design by Matt Jefferies, one of his first ideas for the original Enterprise from 1964. The concept was also considered for a non-Trek series that Gene Roddenberry planned around 1970. A backlit depiction of this vessel was included to a display on the recreation deck of the USS Enterprise in "Star Trek: The Motion Picture", together with other ships named Enterprise that Decker shows to the Ilia probe. In the bar in ENT: "First Flight" we can see a reproduction of this image from the same angle. Considering that it is shown there as a tribute to test pilots, the ship is very likely supposed to still exist in the Star Trek Universe. The Star Trek Encyclopedia II depicts a new rendition of the ship from a different angle.
2) The strange ring structure was originally supposed to simulate gravity (much like in "2001: A Space Odyssey"). Anyway, with the assumption that it is an annular warp drive, it still doesn't fit into the Federation design lineage. The ship, however, looks akin to the Vulcan Surak class. It is possible that humans acquired one or more Vulcan ships, which would explain the name "Enterprise". It is also possible that it is a prototype ship built by humans, inspired by Vulcan technology. If this is so, the project was apparently a failure, considering that all known later Federation starship designs have the familiar nacelles again.
3) The non-canon Space Flight Chronology states that the USS Enterprise, Declaration class, is a 300m long starliner for 800 passengers. As Paul Cargile pointed out some time ago, this is complete bogus. At 300m, the inhabitable section would be smaller than the Defiant, and imagine 800 people in there on a long interstellar journey! In the book the ship looks somewhat different and is a tad more detailed than the versions that appeared on screen.
4) Rick Sternbach said about the design at TrekBBS: "My only exposure to that design was back in 1978 when I was on ST:TMP. The script called for a few ships called Enterprise to be exhibited in the Rec Room. Gene Roddenberry asked me to stop by his office, where he gave me a copy of the painting of the ship, printed as a poster. He asked me to make that design one of the five or so ships in the Rec Room alcove. So who was I to say no? ... As to how that ship fits into Trek history, I really don't have anything intelligent to say. I don't know much about it, never speculated about uh, specs, and have no real opinion about whether it was a civvie vessel or a Federation vessel or just what it was. I also have no additional drawings or even the poster I worked from in 1978. You'll all have to carry on as best you can to shoehorn it into the lore."
5) The name and registry is "USS Enterprise XCV 330". As the ship must have been launched prior to the foundation of the Federation (more precisely any time before 2143, the year of "First Flight"), "USS" may stand for "United States Ship".
