Check to see if it is a back clamp for the feet, if is then you would either need it change it out to the side clamp or be restricted to the old back clamp attachments.
Is the side plate taped in place? If I remember correctly this slides into place from the machine side not from the outer edge in, as most people try to do, so they just tape into place.
I would also go $100.00 for it.
I hope you get it, my 66-1 makes the best stitch I have ever seen.