
In the last STAR WARS film, REVENGE OF THE SITH, two ships from the Original Trilogy make an appearance.....Or DO they?

Above is a picture of the Tantive IV from ANH

Above is a picture of Bail Organa's blockade runner from ROTS
Evident is the fact that the two ships are not identical; the blockade runner from ROTS is shorter, and many surface details are inconsistent with the blockade runner from ANH. A rundown:
The hull lacks the square feature that all the engines attach to.
The hull lacks the short segment connecting the square to the lateral docking segment.
The long fins are radically shortened and not canted at the same angle - much shallower and the lower fin has structural infill (the hangar area).
The "wedge" section is completely different in size, plan view, front and stern views and elevation view.
The escape pods are different sizes, shapes and locations
The (former)gun turrets are located on a tube that is too large in diameter and incorrectly located vs. the neck and the wedge.
The lower neck has an undercarriage bay where the fin should be.

Two drawings of the blockade runner "Tantive IV" from ANH (click to see full size). These incredibly detailed drawings were drawn by Mike Marincic (courtesy, Robert Brown).

Above is a basic schematic depiction of Bail Organa's blockade runner from ROTS drawn by Martyn Griffiths (click to see full size).
Historian Anthony Tully has an interesting take on the subject:
Personally, it makes more sense to assume either the ship is the Tantive IV, and underwent a very thorough naval-style overhaul, the kind that can change length and beam (they do happen), or the vessel is an immediate predecessor of the Tantive of ANH. `IV' probably is not a series number. Remember, the vessel is probably named for a planet, and if so the SHIP NUMBER WILL NOT CHANGE. That's in fact a serious mistake Star Trek publications often make—there would never be a number after the name—you are supposed to "know" if its the fifth or seventh, etc. Our nuclear carrier Enterprise is not called "Enterprise-N" to tell the public its not the Enterprise of WW II. Its fine for Star Trek, but the idea of an Enterprise-A or C is not naval parlance. If the blockade runner is named for say, the fourth world of the Tantive system, then its a proper name and would never change no matter how many incarnations the vessel goes through. Much like a British vessel named King George V wouldn't change if they built another.
Unfortunately, the EU has claimed that this ship is the "Tantive IV" from ANH. However, it wasn't MEANT to be the same ship. A quote from Cinefex #102 confirms this:
pg. 68: "One of the most fun things about this show was linking the first trilogy to this one," said animation director Rob Coleman, "introducing designs that make more of an appearance in A New Hope." The cruiser flown by Bail Organa, for example, is a predesessor to the rebel blockade runner featured in the first shot of the original Star Wars. "It looks a little bit different, but you can tell it comes from the same factory."
Strangely, ILM seems to have digitized the electronic Blockade Runner toy from Kenner!
The alleged "Millennium Falcon" in ROTS seems to have suffered from the same trip to the toy box. A close-up picture of the YT-1300 freighter purported to be the Falcon, from the "Making Of Revenge Of The Sith" has the exact dimensions of the incorrectly proportioned Falcon products available to fans.

This is a photograph from the actual Millennium Falcon prop used in the Original Trilogy

This is the alleged Millennium Falcon from ROTS. Note the thick 'edge trenches', characteristic of many inaccurate licensed products such as the old MPC/Ertl model kit and Code 3 replica.

Above is a comparison picture between the inaccurate ERTL model (on top) with the accurized version below.

This is the Code 3 Falcon again with the inaccurate side panels
So, how to fix these anomalies? The Millennium Falcon gaffe isn't bad enough to warrant any fix. Thankfully, it's far enough away from the camera, and filmed at an angle that doesn't blatantly betray the fact that it isn't the same ship from the classic trilogy. The "Tantive IV" however, has many "hero" scenes in ROTS. Again, while it's never named the "Tantive IV" in the movie, the novelization, ROTS Visual Dictionary, and Star Wars Insider identify it by that name.
One fix for this is simply that Bail Organa likes the name, and adorns his personal starships with that moniker which may refer to a long ago battle on the planet Tantive IV. Wanted by Cracken, an old RPG supplement from West End Games, includes a battle on the planet Tantive V between the New Republic and the forces of Admiral Drommel.
Well, that settles that!
Martyn Griffiths
Anthony Tully
Robert Brown
Tim Ketzer