Special thanks to Michel van Loon, creator of the afregister.org website. A nonprofit organization that is trying to create a database with surviving armor from around the world. He was able to provide clarity on some of these Shermans in South Africa.
I will do some reading and see whether I can provide more information on this variant of Sherman. The other two examples that I wish to mention are the Sherman Firefly at the National Museum of Military History in Saxonwold.
(Update 14/06/2013. This is technically a Sherman that did not exist. It is an M4 Firefly hull with a 105mm turret on it)
It is evident though, that the many variants of Sherman out there can really be confusing, and when next I visit the War Museum I will see what the plaques associated with the two Shermans say. Hopefully I will be able to track down some of the others in and around Gauteng while I am about it, although they will probably just leave more questions than answers.