The De Buys/Beyleveld inter-family "group" marriages was not the first such mix-up in the De Buys family history.
Johannes Gerhardus Hendrikus Pertorius and his second wife, Johanna Christa Jacoba Coetzee had three children, Gert Cornelius Adriaan (born 1833), Johanna Christina Jacoba (born 1834) and Catharina Maria (born 1835, died 1875).
On 1 October 1850 Catharina Maria Pretorius married Petrus Johannes Du Buis (Born 16/11/1832, died 13/12/1880) in Potchefstroom. They had two boys and two girls.
On 07 February 1853 Gert Cornelius Adriaan Pretorius married Maria Petronella Du Buis (Petrus' sister) (Maria was born in 1836 and died in 1900). They had one daughter.
On 25 November 1954 Johanna Christina Jacoba Pretorius (who was known as Francina, probably to avoid being confused with her mother who had the same names) married Joseph Coenraad Du Buis (born 15/04/1834) in Potchefstroom. Joseph was the brother of Petrus and Maria). Joseph and Francina had three daughters and two sons.
Joseph Coenraad and Johannes Pretorius (the father of the children who all married Du Buis') were said to have had a very close relationship.
Joseph Coenraad and my great-great-great-grandfather, Coenraad Willem De Buys, were brothers.
I'm fairly sure that the Johannes Gerhardus Hendrica Jeremias De Buys listed in the Anglo-Boer War Concentration Camp records was a son of either Joseph Coenraad Du Buis or Petrus Johannes Du Buis, but I have no way of linking them. (Both of them had sons with very similar names) (If you have any information on this please feel free to leave a comment!)