Mid to late 70's, a bit earlier if you include the Schwinn World Voyageur (Japanese Paramount, which is a FINE bicycle IMO)

On the hi-ten topic: I have a late 70's Sekai that is fantastic riding and is a hi-ten frame. Not as lively as some of my other 4130/Columbus/Reynolds/Tange/Tru Temper bikes, but it still holds it own, and is surprisingly light. Also, one must consider the geometry which I think also factors into "road feel" when it comes to these early domestic market Japanese bikes.

The fit and finish of these bicycles compared to some other bikes, in my experience, was much more consistent.

Finally, isn't this topic (like many others on BF) very subjective? Not trying to insight an argument, just an honest question.