According to a press release, on 6 November 2013 the South African Reserve Bank revised the banknote family launched exactly a year prior by adding Omron rings (or as the SARB calls them, “little dots”) on the fronts and backs of all denominations (visible in the watermark areas on the images above). The existing Mandela notes will continue to circulate in parallel with the new ones. Omron rings appear to be randomly positioned design elements, but in fact can be detected by modern computer hardware and software to restrict the editing and printing of banknote images.
Courtesy of Ruan de Witt.