Not just citations....
the Lear 45 had just such a flange added to the rudder, a few years after it entered service. A service bulletion. I don't remember exactly, but I think it also extended onto a portion of the rudder tab as well, not sure. It was not along the full length of the rudder, maybe for half of it I'm guessing. IIRC, this allowed the aircraft to dispatch with the yaw damper inop. I do not know if there are additonal operating limitations if dispatched inop. so I assume the flange contributed to the yaw (directional) stability.
Sounds very similar. I can try get a picture.