Like it says, what is that little piece up in there that holds the bolt in?
What would you do to hold the trim tab anode on if that little piece up in there was not there any longer?
Just a reminder: If you drop your lower to change the impeller, do not leave the allen wrench sticking up out of that hole and tilt the drive up. Especially if you have a swim platform. You'll likely pop that little bolt holding thingy, and maybe the metal that holds it in, straight out the bottom of the trim tab anode cavity.