There are booster springs for a 48 (and I assume 46 & 47) converts. There is a picture in the 41 - 48 service bulletins, page 317. It does not show where the bottom is mounted. I installed then in my 48.
Since the picture showed what I thought to be be straight up and down with the top up, I assumed there was a bracket down below. Any bracket that should have been was missing, so I made one. I don't know if the stretch is right, but it's been working ok for the last 30 years.