Some 15s thread right to left and some left to right. The easiest way to check which way it threads is to look at the last guide before the needle. Does it pull the thread to the left or the right? If to the left, it threads left to right. If to the right, it threads right to left. Alternatively, you can also look at the needlebar. The flat of the needle will always set against the flat side of the needlebar.