I took my own pictures of the pocket stop and an outside view of the bolt. Other than Maria's pictures, there are no other reference pictures that I have seen of this area. This may be helpful information for those looking at older TM during the transition years.
In the pocket stop you can see two layers of aluminum, one thicker than the other. The thinner one is the aluminum skin, the inner one supports the lift arm.
Presumably, the weight of the shell rides momentarily on this plate while the shell is being raised or lowered. As Bill said, the shell is like an an aluminum box, so this plate will be securely attached to the metal framing, making it part of the entire frame.
From the side, you can see the inner aluminum plate behind the peeled outer skin. The hole here is slightly elongated.
Presumably, IF the hole in the inner plate become too elongated, a replacement plate could be inserted and firmly secured to the aluminum frame member visible in the first picture. Given that this trailer is 16 years old, and it has lasted this long so far, I doubt it will be a problem.
Removing the bolt to insert a fender washer may be a good idea, but I would only use stainless steel to prevent electyrolytic action with the aluminum.