I've seen quite a few of these rings on these coins. Some are deep in the coin similar to roller damage but most just show up like this. Some more examples: https://coins.ha.com/itm/south-afri...1910-61321.s?ic4=GalleryView-Thumbnail-071515 https://coins.ha.com/itm/south-afri...3054-31384.s?ic4=GalleryView-Thumbnail-071515 https://coins.ha.com/itm/south-afri...1708-64256.s?ic4=GalleryView-Thumbnail-071515 https://coins.ha.com/itm/south-afri...3053-38294.s?ic4=GalleryView-Thumbnail-071515
I'm not convinced that's all. This is consistent across this series which means it's something to do with the production process.
Are the tings influenced by the texture of the coin at all? I can't tell. Is it possible that something was done to the surface of the blanks before minting?
It is not just South African coins that have rings. Here is a Maria Theresa Thaler probably minted within the last 100 years: Maria Theresa Thaler with obverse ring