I'm with you, Alan, the engine and transmission are original to the car. Could the engine/trans both have been swapped out after the build date of the car. I'd say No.
I dont subscribe to your theory about the build date mixup. First, engines built for warranty replacement are shipped to a parts warehouse. They aren't stored at the engine plant. So I don't believe this engine could have been taken from an in-plant stock of replacement engines. Second, the build date on the body tag isn't really the car's build date - it's the body FRAME date, i.e. the date the body was framed. My guess on this is that the car was scheduled with the expectation that police engines would arrive in the plant in time for assembly into the car when it reached that point in the plant. Each assembly plant has a holding area for completed, painted, untrimmed bodies. If word reached the plant that the police engines wouldn't be available by a specific date, the body shell would be held in this area until word was received that the engines had arrived. Then the shell would have been released back into the production line to be finished. So, given that your engine's build date is 12 days later than the scheduled body frame date, my gut tells me this car's actual "build" date, when the car came off the end of the line, would have been another 3-4 days later yet. Factor in material handling at the engine plant, transportation to the assembly plant, material handling to unload the truck or rail car that brought the engine, paperwork processing to let the scheduling people know that the engine had arrived, etc.,etc., and I can see an easy additional 3-4 days, if not more.