I can't find anything definitive about what Datsun/Nissan was doing but my understanding was that clear coats weren't used in high volume until the 80's and mostly on high end cars. I remember guys getting clear coats on their hot rods in the late 70's but that was a custom paint job.
This article suggests that the late 70's/80's was when they really came in to high usage.
https://www.paint.org/coatingstech-magazine/articles/brief-history-automotive-coatings-technology/
"As previously discussed, thermoplastic acrylic lacquer automotive coatings, given their excellent appearance, were the major automotive topcoat used in the 1950-70s. However, these lacquer topcoats did have one significant drawback: they had weak exterior durability. After about one to two years’ exposure, the coatings would begin to degrade, and aggressive waxing was needed to “bring back the shine” of these systems. By the 1980s, the automotive manufacturers were requesting better durability for automotive topcoats, as consumers were now expecting their cars to last at least five years, and they wanted the car to continue to look like it did when they first saw it in the showroom. "
This Toyota page shows a clear on a solid as a one step/one bake process in 1973 but no clear top coat again until 1990.
https://www.toyota-global.com/company/history_of_toyota/75years/data/automotive_business/products_technology/technology_development/materials/painting/index.html