Many riders liked the shape of the B66 S, but didn't like the 4-wire clamp. If you have a modern seatpost with a built-in clamp, this is an inconvenience, and previously the only options were to spend 12 bucks for a Seat Sandwich, or to use an old style "pipe" type seatpost. The B67 series solves this problem. They have the same top as the B66 standard, but the undercarriage is the modern 2 rail design, compatible with all high-quality seatposts.
All of the B66 and B67 models feature bag loops to permit the use of a traditional English-style touring bag.
The B67"s" is considered a "women's" saddle.
This distinction is not as clear-cut as might be supposed. It is generally understood that women, on average, require a somewhat wider saddle than the average man. There is a basic biological reason for this: women, on average, have wider hips than men, so their ischial tuberosities (sit bones) are farther apart.
It is also customary for women's saddles to be shorter than those marketed to men. I'm not convinced that there's any biological reason for this; rather, I believe that the custom of making women's saddles shorter is for the sake of accommodating a skirt more easily. Women who don't plan to ride in a skirt should not rule out "men's" saddles, particularly the wider models.