These photographs of the South African Heavy Artillery Memorial at the Zoo were taken on 20 August 2011. Special thanks to Carl Hoehler for the information on where this memorial can be found, as well as additional information on the 6 guns that made up the memorials to the South African Heavy Artillery. Of special mention is the worker at the zoo called Kevin who assisted me in getting these photographs.
This 6-inch 26-cwt howitzers is one of 6 brought back from France and Flanders to be part of the memorials to the South African heavy Artillery that were established in major centres in South Africa. It is currently in the grounds of the Johannesburg Zoo. Like its counterpart in Port Elizabeth, part of the memorial is a dedication plaque:

Erected by the Officers, NCO's and Men of the South African Heavy Artillery in Memory of their Comrades who fell in the Great War 1914-1918

The inscription is followed by the names and ranks of the 167 men who died in the Great War. The gun has been in place at the zoo since 1920, and was restored in 2007.
It can be found at Google Earth Co-ordinates 26° 10.111'S   28° 2.039'E or Download the KMZ file.
The 6 Memorials to the Heavy Artillery can be found in: (Open in new page)
71st (Transvaal) Siege Battery: Johannesburg Zoo (Restored 2007)
72nd (Griqualand West) Siege Battery: Clyde N Terry Museum Kimberley
73rd (Cape) Siege Battery: Company Gardens Cape Town
74th (Eastern Province) Siege Battery: Port Elizabeth. Gun removed to the National Museum Bloemfontein to be restored
75th (Natal) Siege Battery: Warriors Gate Durban
125th (Transvaal) Siege Battery: Near the Union Buildings Pretoria