Yup, 'Pristine' condition. We know that the Overlord-class DropShip that replaced the Dictator showed up in the 2760s, so maybe a unit had their original Dictators replaced and put into mothballs before the Periphery uprising.
Of course, we're not necessarily arguing that the Blakists built the facility, more like found/activated a facility.
Lets say that it was some sort of storage facility/boneyard. Then it would stand to reason as well that there are some sort of decommissioning/recommissioning facilities as well I'd think. Not a full production line or anything, but enough to pull the Dictators out of mothballs, perform maintenance and repair, and to refit them to put them back into usage.
Now, if there's a minor facility like that, then it might be enough for some Blakist to tinker with refitting a Vengeance as well. Then if the Magistracy captures it, then they have a Vengeance, but no way to actually mass produce it (the yard can only do slow refits) and any data they have on it is more suited for refitting than production. So they take the Vengeance to the Capellans, tell them to pour over it and figure out how to mass produce it.
It sort of fits. A boneyard/refit facility would make sense, rather than a full production line.
Of course, you've got to ask about the timing as well. The Magistracy started to deploy the Dictators around 3070. That's pretty quick after the Jihad started and when they're still dealing with Blakists..is it likely they found a Blakist refit yard that quickly? Why would the Blakists be refitting the Dictators back to Star League standards (actually, more like Reunification War standards). If it were a Magistracy project all along, that might make more sense.
Lets say the Canopians stumble across an old, Star League era boneyard with minor refit/repair facilities, sometime in the 3060's. They find Dictators there, left over from the Star League (the League didn't take them when they pulled out because it would've taken too long to recommission them, time they didn't have once Amaris struck Terra, and they didn't want to hang around in potentially dangerous space).
They can always use DropShips, so they refurbish the facility, and start to recommission the Dictators. They don't have lots and lots of experience designing/building new DropShips, so they figure they'll put them back to their original configuration. No need to design anything new, no need to make Part D fit into the tabs for Part A. Keep it Simple.
Then at some point, maybe they start to run out of Dictators to bring back into service. So someone says "Lets push the boundary" and they start to slowly reconfigure a single Vengeance with advanced technology, inside and out.
It could work. The question is the details :)
