crocee,
First the dash number is a variation. A good example is the 15-91 is the old black machine style with the potted motor.
Some dash numbers do not indicate any differences in the machine, but indicate that it was in a treadle or a HC or a knee bar case.
For a good read up on machines go to ISMACS {
http://www.ismacs.net/home.html } and go to the Singer sections. There is tons of info on manufacturing codes ( the letter K for instance ) and other things.
As for the clones, they are not true clones. They are based on the Singer model 15s with modifications and updates.
Joe