HI,
I can't seem to get a measurement of my sitbones that would be accurate in any meaningful way.
I can tell you that my hips are about 42", measured standing up and around the sitbone area. I have a definite pear shape.
The B17 "did" fit me comfortably, but I wanted that wee extra width of the B68 for added insurance.

As to nose length- I don't notice any difference for me in either the shorter "S" nose or the longer non-"S" nose on the brooks saddles. It seems to make no difference to me. The S models however do have shorter rails because of their shorter noses, which means they have less for/aft adjustability than the non-S rails. They can't get shoved quite as far back on the seat post.