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6) How do we know that was the Defiant? Could it have just been a Defiant-class starship launched in that same window of time when Equinox and Voyager had been?

The ship does have USS Defiant NX-74205 on its hull, though admittedly that doesn't mean much since both Defiant class ships sent to retake the Prometheus have that stamped on their hulls. Perhaps both the Defiant and the Sao Paulo were sent together?

At the very least, the two officers who beam aboard the Prometheus bridge, as I mention in my earlier post about the phaser rifles were played by regular background extras from DS9, certainly lending weight to the argument that the Defiant really is one of those ships.