OK, "the Middle Stone Age in Southern Africa ", hmmmnn...
According to the anthropologist Carleton S. Coon in his mid-20th Century book and intellectual tour de force, The Origin of Races, southern Africa (the vast region south of the Limpopo River) was inhabited solely by the Capoid people before Europeans arrived. Capoid ethnic people survive today as the Bushmen of the Kalahari.
When was the MSA supposed to have ended for the precolonial Bushmen? How have their tools changed from the tools you found on the beach?
That book would be rather dated. Capoid is a archaic term. The bushmen are now referred to most often as the Khoisan.
They are an LSA stone age culture creating much finer and smaller stone implements than MSA tools.
LSA is characterized not only by minute flake lithics, but also ground lithics, beading, rock etching and painting.
There were a number of waves of iron age peoples that migrated into Southern Africa before the Europeans arrived. Some annihilation of the incumbents took place via displacement and interbreeding. These migrations occurred in the moister east and progressed southward, the Khoisan that remained practicing their hunter gatherer lifestyles were force to the drier western parts and some have survived in these parts relatively unchanged till less than a century ago.
They are not considered the ones that made MSA tools.