Hello.
Inside the Silver Efex Pro 3 (last version, from nik version 4.3.2) interface (hosted in PS) the “Compare” button displays:
image 1) the image of the current PS layer with the default Silver Efex filter already applied (neutral)
image 2) the image with the selected Silver Efex filter applied.
One would expect that in “image 1” there is the original image of the PS layer, without the default Silver Efex filter already applied.