The S15 was the unfinished version of the S10. you can read more about the history of this spinning wheel here.
The S15 is the unfinished brother of the S10, produced from 1982 to 2001. The S15 was also available as a kit from 1984 to 1992. From 1998 to 2001, the S15 was available with double-treadle (S15DT) and again as a kit (S16)
Spare parts from the S10 fit the S15. If you need a bobbin or flyer, you can choose between that of the S10C IrT (lacquered) and S17 (unfinished and therefore you will need to finish it yourself).