Hello Archdeacon, and welcome to Seven Forums.
You can look in the registry at the location below.
Once there, look at each long number CLSID key in the left pane under NameSpace until you see the same TweakUI entry that displays in the Control Panel in the right pane for (Default). If you spot it, then you can just right click on and delete the long number CLSID key for it.
Hope this helps,