Mach buffet in the 737-200 will occur below MMO. As the aircraft speed increases in the Mach regime airflow seperates from the wing and this causes turbulence that the horizontal stabilizer encounters causing the buffet. This buffet is benign.
The BAC 111 would, in an extreme overspeed condition, exhibits rudder reversal. I would think that it would be nearly impossible to force this particular aircraft into this situation.
The USAir BAC 111 simulator had this programmed into its flight characteristics. As I recall it required a signifigant amount of will power to force the airspeed and pitch down required to get rudder reversal.