That is the pivot point of the rudders. I have a pic somewhere that shows the gap between them and the main fin and you can see the shaft.

Edit to add pic. This isn't the best pic. It's just from a quick shot I made when I took my daughter to Huntsville a few years ago. I seem to have misplaced my files with details of the MR. Still, you can see the gap on the fin to the right, and you can see the shaft and its entry point through the plate on the fin closest to the camera.

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