I could have sworn I had heard that while the Z is manufactured in Japan once they arrive in California Nissan is operating a small shop where a body panel or two is then added. This was done as a means of getting around the current low tariff. But I could be totally wrong.
it just says final assembly on window sticker because as the ports they add small things for example if you buy a heritage they put the bumper wheels and decals on at the port the car comes from japan with the standard bumper and performance wheels. Zs come in at ports both east and west coast and this process happens at all the ports
they also prep the cars by adding floor mats and under spoilers there so the dealer can install them once put on a truck