Over the past couple of weeks the topic of trying to find back clamping feet for the old Singer 66s has come up. I just ran across the TreadleOn site again, and they have many great links you can read, one of which is how to convert a presser bar to accept side clamping feet. Here is the list of topics, "converting back clamp feet" is the fourth topic down.
http://www.treadleon.net/sewingmachineshop/index.html