Mandela was born in 1918, and many countries issued stamps to mark centenary of his birth last year.
Minisheet on the right was one of many issued to celebrate that event, but this one is joint issue with Germany, released on 18.7.2018. Other stamps on cover:
The 300th Anniversary of Stellenbosch (Oldest Town in South Africa), issued on 8.11.1979.
Moore's Crinum, issued on 1.7.2013.
Eland (from set Tugboats), issued on 13.5.1994.
The 100th Anniversary of the Battle of Amajuba, issued on 27.2.1981.
Thank you Willy!