I'm pretty familiar with normal wood knots, that look like dark solid circles. Are these knots as well, from far away they look like knots, but up close they look more like swirls. However I can't think what else they would be.
Yep, they're knots. It's not a defect, it's wood! I wouldn't worry about it as long as the knots are small and sound, especially since it's on the back.
I saw a display of highly prized 17th century Cremona violins at the Metropolitan Museum of Art in NYC. These things cost even more than a Moore Bettah! There was a Guarneri or Amati, I don't remember which, that had a knot the size of a quarter in the lower bout of the top that looked like it was about to fall out. Made me feel better about the little defects that I fuss over.