I understand that the US Government arranged for Japan to get design information for sewing machine models after WWII (1945) that the patents of which had expired. If so, then I would say that it was probably the Singer Model 15-30 or a similar model in the class that was expired by that time, and the design adapted by the Japanese makers. Singer was still making the Singer Model 15-91 at least up until the Centennial (1951), and the basic design had been changed to the right-to-left threading before then, so Singer probably didn’t care about the old design that much by that time. I’m just guessing, so don’t take my word for it, but it seems like the majority of Japanese “clones” tend to thread left-to-right like the old Singer obsolete 15-30.
CD in Oklahoma