Utah,
Looking at your photos, I believe the rib extension has moved to the rear. It may have moved a little and may still be tight, but it has moved, as the rib matting lines no longer mate up with the lines in the top of the rib extension, and looking closely, the sides of the rib extension where it comes out of the barrel face, look to be above the surface of the barrel face. Now it may have moved and still lock up properly, but I doubt it. If it is a "dire" as I think, the only way to repair it, is remove the ribs and all, which are soft soldered. Then with proper heat, remove the rib extension, clean, re flux and silver solder the extension back in place, then reassemble the barrel set. No small job.
When building the barrels, after the barrels are silver soldered together the seat for the rib extension is cut (maybe like 3/16 in or so) and the extension silver soldered in place. AH Fox barrels were built in a similar fashion.
If one tries to re silver solder the extension back where it should be, without cleaning the joint, one stands a good chance of not getting a good joint, and compromising the soft solder joint of the top rib, In my opinion. It generally is worth doing right. IMO
