15x1 SP: HAx1 SP:

This is a special version of the 15x1 for stretch or elastic materials and tightly woven knits. 
It is made to higher standards; it has a snake’s head shape due to a combination of a slightly bulged eye and a slightly reduced blade above the eye.  The bulged eye opens a slightly larger space in the material and the thinner blade above the eye reduces contact between the material and the needle. 
This reduces needle heat-up and prevents skipped stitches. 
It is also available in a light ball point to separate the fibres rather than cutting them, avoiding needle holes in the fabric.