Early 1970s. Nixon visited China in 1972. Don't forget that China had to industrialize first - it was initially importing equipment from the US. Imports into the US at the time were more likely to come from Japan, Taiwan and Korea. Seems a to be a bit too late to explain the timeline, but it could be a factor - OTOH, given that China's workforce is no longer expanding then your theory would predict that wages will now stop stagnating but that does not seem to be the case?