Milt
I have never been given an explanation by anyone I would believe (Kingston D.O.for example) but here are some thoughts
I always assumed that given the depth of the boundary layer by the time you get back there and the tendency for the shock waves on the tail to wander about when decelerating from the dive then the rudder could build up quite a deflection before it stuck out far enough to get blown back and so thrashed from side to side when it did eventually find some high velocity air.
Thickening up the trailing edge of controls was a fashionable way of improving their effectivness at high IMN in those days..
Finally if anything wants to oscillate making it a bit asymmetric often changes things enough to damp the oscillation
But I don’t really know how to answer your question Milt. At the time doing these refurbished Hunter prod airtests before they went overseas was a major distraction to the job in hand which was trying to get the jump jet working properly.
JF