I looked but was unable to find any pics of the inside of a tailgate for a 57- 58 wagon. The trim pieces are the 4 straight stainless that screw to the inside of the tailgate. They look to have been designed as some kind of runner for loading perhaps. Anyhow I am looking for a set. If you ever opened a 57 wagon , you couldn't help but notice them.
I have a set that came in the parts box I got with my 57 but they are ruff. Since I have no plans to go back to original these may be available. I think I will make a set the is more robust. I will send you a PM when I figure out what is going soon.