Nikon does not have a "dual production line." What they do have is U.S. Subsidiary incorporated (last time I looked) in New York State.
Nikon Japan sells a portion of its output to Nikon USA, which in turn distributes it within the United States. Lenses meant for import into the United States often have serial numbers with a US prefix. In the past, Nikon also badged some film cameras differently for the United States. But outside of such cosmetic differences the gear itself is identical.
At least currently, it's legal for other companies to buy Nikon gear outside the United States and import it themselves. This may change depending on the outcome and ruling of a case currently before the U.S. Supreme Court. Nikon USA doesn't get it's "cut" on such sales; they consider the "parallel importers" to be competitors rather than partners. As a result they refuse to provide support or service for gray market gear, even on a fee-for-service basis.
"There is no real magic in photography, just the sloppy intersection of physics and art." — Kirk Tuck