I think in general, groupings of DSO's were assigned to a unique assembly plant, but due to certain circumstances, DSO's would get a car built at a different assembly plant.
Interestingly, I had a '67 convertible, nightmist blue with std. blue interior, has a San Jose VIN and DSO of twin cities, MN. Helped a friend buy a different '67 convertible, also nightmist blue with std. blue interior, Dearborn made and twin cities, MN DSO. The cars were both built around July '67 if I recall correctly.