I've both, the softener have little clear flat patches on the surface of the plastic lens. The diffuser have mild rough texture on the entire surface of the glass. They look different but the effect is similar.
You are absolute right! Nikon softener filter is called "Soft 1".
Actually, I "think" I do see a difference between Nikon Soft 1 filter and B+W Diffuser. Nikon soft 1 is a solid piece of "blur" glass while the B+W's has round concave min-lens on its gass surface. Digital print under magnification, the Nikon's has a homogenuous soften effect throughout the picture. For the B+W's, its' only at regular intervals then has the soften effects. But overall, to the naked eyes when holding both photos, they looked the same.
Kho King, there are ways to create DIY soft filter with creative effect, e.g.:
Use clear-colored nail-enamel, drop it randomly on a plate of glass (last time I use Cokin blank).
Another creative soft-filter:
Use vaseline, smear it over a plate of glass, it will create effect which Photoshop can't do. This technique was last time used by one of the famous wedding photographer in U.S.
The B+W Carl Zeiss Softar 1 cost me $450 the last time I bought it. It's actually made of a plastic material, not glass. I don't "feel" any difference between Nikon Soft 1 and this. I sold the CZ's off just a few months back to a sucker who went aftr branded stuffs.