Based on the features, I would say it was very late 1970s. More likely, it's the Superbe model, which was their top line model at that time. It was named after the SunTour Superbe components that it was equipped with. This accounts for the SunTour dropouts. Nishiki was manufactured by Kawamura, so they are essentially the same.

By the way, may I ask what the serial number is? Nishiki is a brand for which I am trying to break the serial number code. Thank-you in advance.