I visited the historic diamond mine at Kimberley as a boy. We were told that the claims at Kimberley were 30 Dutch feet square, which equated to 31 English feet. It seems the standard size for diamond claims in South Africa was 30 feet square.
In fact, it seems that was not quite correct. The Kimberley and de Beers claims actually used the Cape Foot, which was:
defined as 1.0330 English feet (and equal to 12.396 English inches, or 0.31485557516 meters)
So, 30 Cape feet = 30.99, or to a close approximation, 31 English feet.
Interestingly, the Wikipedia article notes that the Cape foot continued in use until 1977 when South Africa adopted the metric system. That kind of thing can happen when multiple standards exist side-by-side.
As Theodore Reunert observed in his Diamonds and Gold in South Africa, published in the 1890s, claims at Bultfontein and Dutoitspan used the English foot as the standard [p42].
It is worth noting that these mines were discovered slightly later than the Kimberley and de Beers mines (the first years of production in these mines were 1870 at Bultfontein, and 1871 at Dutoitspan), so they were less developed when the diamond mines were annexed under the proclamations issued by Sir Henry Barkly in October 1871. This might therefore account for the use of the English foot in setting out claims, rather than the Cape foot used previously.
The annexation of the diamond fields is described in some detail in chapter 9 of Sir Charles Paynton's The diamond diggings of South Africa. A personal and practical account [pp51-56], which was published in 1872.