The original Guardian, the one I believe to be in possession of the New Republic, was captured by the New Republic from Admiral Drommel around the time of the Corellian Crisis, and rebuilt over a period of an year, after which it was added to the third fleet when it was sent out with the 5th fleet to engage Pelleaon, and reduce his Empire to the mere 8 sectors of now.
Drayson believes that the Guardian in the novels was one that the Empire built during it's peace time with the NR, or one that was acquired through the moffs via like Reaper was. There was an SSD in possession of the Empire in the beginning of Destiney's Way, that I believe was not the Guardian. So, who owned the SSD Guardian?