The 15-90 series (11:00 finger as viewed from the solid side) of cases thread so the the bobbin is unwinding counterclockwise. The other series (1:00 finger) unwind with the bobbin spinning clockwise.
Ahhhh, I see said the blind man. So the 15-91 is opposite of the others when it comes to the bobbin. I'll do my best to remember that.
Joe