Caribe81 posted some pix of her B17 "S" saddle here:
http://forums.teamestrogen.com/showt...t=14586&page=3
There is one shot that shows the straight (useable) section of the under rails with the clamp on. Keep in mind that the NON-S B17 has much longer straight section of rail- the part you can slide back and forth to adjust.
The B68 non-S has the longer rails as well. The Brooks S models are shorter and their rails are shorter.