Hi Paul, yes the aerodynamic balance tab did look exactly like the one you have drawn.
I cut it off and glued it to the fin. It looked similar to what it had done before except there was more fin area and less rudder.
I can't remember what the model name was. It was similar to an Extra but it was about 1980 and the name escapes me.
I did wonder if the problem was helped by area being removed that was high up above the centre line. Maybe right rudder was rolling it left a little bit and the lower rudder helped.
I also had a Club 20 pylon racer with a large V tail that wobbled during turns.
I cut the area down and that cured that. Never a definite answer is there?
Jim