I have a charles Johannes duPLESSIS born 10 Nov 1812 and baptised 31 Jan 1813 in Graaff-Reinett who had two wives.
He first married Johanna Cathatina Adriana VENTER on 8 Aug 1830 also in Graaff-Reinett.
He second married Johanna Hendrina Aletta YZELE but I don't know when.
He had 11 children and his 9th child was Charl Johannes duPLESSIS. I have no birth date for him and no more info. I also have no death date for the first wife so he could be the child of either one. If his father was married first in 1830 and he was the ninth child he COULD have been born around the right time for you. The death cert of the father would say.
Julie Whitton (nee duPLESSIS)