Lisa - the B17S is 177mm wide.

My sit bones are about 180mm outer edge to outer edge, so I doubt a B17S would be as comfy for me as the B67. (I've got the wide version of sit bones)

The B66-67-68 have a very abrupt change from wide part to nose. I'm one of those folks whose hip angle makes their thighs chafe like mad on a "pear" shaped seat, so the "T" shape of the B67 is heavenly. No chafing.

The nose of the B67 is the same width as the nose of the B17 and it's relatives. Very narrow.