I am wondering if one could comsider that Death Star at the end of ROTS the supposed "Death Star prototype," as discussed in Kevin J. Anderson's "Jedi Search" and "Champions of the Force." In the books, it is explained that a prototype was basically built before the actual first death star, and was hidden away in an unnown sector of the galaxy, where it was later destroyed by the New Republic. Now, it's interesting that these books were written a few years before ROTS, yet provide an excellent explanation for the 19 years it took to finish the actual death star. If you combine this explanation given by Anderson (which is considered canon by the way), with that of Lucas in the DVD commentary of "union disputes and supply problems," I think it would be fairly reasonable to say that that WAS in fact the prototype. See, originally, Anderson wrote the deal with the prototype in his books in an attempt to explain away the events in the Star Tours ride. He was attempting to make it canon, and he did. However, it works well now for ROTS as well. Until the 2007 book on the creation of the Death Star is produced (which SW.com reported was in the works a few months back), we won't have a complete answer.