The 47-50 seat frame works off a continuous cable that winds across pulleys (underside of the frame). The slides are different then the 51-55 style which uses a release rod.
https://www.flickr.com/photos/advance-design_parts_co/29437000195/in/album-72157631868452754/
What you are missing are two small square plates that bolt to the hole in your picture. These small squares secure the cable to the slides when the bolts are tightened.