Hi Mark - what a wonderful antique gold ring, and with a history attached! The term "antique" is usually a relative reference to the fact that an item is old, but how old an item must be before it's defined as antique varies from country to country.
I used the term "antique" loosely above but generally prefer to leave its characterization as an "antique" to others.
The interesting thing is, the purer the gold the easier it should be to mark it. Now, thankfully you will not have to do the biting gold test to help verify if the gold ring is in fact 22 carat gold as your wedding ring has been hallmarked. Sigh of relief!
There is an age-old tradition of biting gold in order to test its authenticity. Whatever you do, don't bite the gold too hard! Although this is certainly not a professional way of examining gold, the bite test should score the gold because gold is a soft metal, as indicated by its score on the Mohs' scale of mineral hardness.
Wedding rings crafted from 22 carat gold are traditional gold weddings rings! For instance, in keeping with British Royal Family tradition, royal wedding rings were crafted from gold from the Clogau St David's mine in Bontddu, North Wales.
Previous royal wedding rings have been made from 22 karat gold from the same mine. Some indeed, from the very same nugget of gold. In keeping with British royal tradition only 22 carat gold was ever used for royal wedding rings.
You mention that your late grandmother was married in South Africa around 1920-25. What a most colorful time to be in South Africa.
Ahhhh.... but the land is beautiful, it's also full of gold.....since the 1880s, South Africa has been the source for a large proportion of the world's gold supply, with about 50% of all gold ever produced having come from South Africa.
The city of Johannesburg located in South Africa was founded as a result of the Witwatersrand Gold Rush of 1886 which resulted in the discovery of some of the largest gold deposits the world has ever seen.
Thank you so much for writing in.... If you have the time, please tell your story (you can attach a photo if you like) on the Your Favorite Jewelry forum.