I forgot to add that none of the 15 clones are rotary, they are occilating hook, all variants and derivatives of Singers model 15. They start to appear just after 1900, at least they become more common then. Among the old straight stitchers there are a few rotary hook models, some US made ones; National, White, etc. They are per definition not 15 clones.
It's hard to date Japanese clones, the variants of them were made well into the 1960s, and can look pretty much identical to yours. Into the 50s you can see some with the tension in front and not on the face plate, some were given a more modern looking body, various ways to adjust feed dog height, turn knob, push buttons, etc.
Last edited by Mickey2; 09-14-2018 at 09:49 AM.