If your watch case matches the 2528 as Ken illustrates above, it would appear in theory you are correct about a replaced or mis-stamped back.
I say that with a grain of salt, as I don't know if all 3 cases you reference have the same back diameter to make them interchangable. Movement and caseback match by date stamp. I would venture the observation that, as the interior case numbers on these are ink stamped that it's possible they (back set) all used the same caseback and the stamp is for repair parts identification purposes? In that line of thinking, it's possible the back was mis-stamped in the factory, or overrun used on the next production line. I would think the s/n would also come into play. Just a logic based thought, as Accutrons aren't my strong point and I'm still learning them.
Drawing on my OCD tendencies, it seems unlikely that someone would go through the trouble to convert a 2600 to a Spaceview using correct parts and then use the back from a second non-Spaceview watch without removing the ink stamp to finish the illusion.
Watch certainly does present as a correct 1968 Spaceview G.