Here are some pics of B67's on different bikes I have, or have had. #1 and #2 are roadified old MTB's for commuting and long rides as well. #3 is a Trek 520 touring bike with North Road bars and a B67. When it was set up like this, I called it a 3-speed on steroids.

You can see the single rail under the saddle that is clamped in a standard, modern type seatpost. The 66 will have two rails and you'll need a seat sandwich to use it with a seatpost like these.

#1
http://i12.tinypic.com/4ub44dv.jpg
#2
http://i16.tinypic.com/4lzewoz.jpg
#3
http://i17.tinypic.com/67e64o7.jpg