I found this on Jim Carter's site and it is for a light installed over the center of the rear doors for increased visibility at night. (See figure 79 and step 4) I did, however, find a thread from 2016 titled "Turn Signals for AD Carryall 1947- 1953." In it, jorb states towards the end of his post that "There is also a reinforcement plate that goes on the inside." I knew I saw something about an inner plate (rather than a pad) somewhere.

It's a small world, but I wouldn't want to paint it.