The portrait currently attached to Bergh-193 when enlarged has anchors on the buttons and also in the embroidery on the collar of his coat and I'm not sure if this is just decoration or suggests some connection to a navy?
There is also an inscription which I can't read that well, but it looks like it has "en 1821" in the second line. Though it is also possible the 2 is a fancy 0?
Otherwise I think it's difficult to definitely judge by the garments alone. The wig in the portrait is probably from an earlier period, but regardless of whether he is in the navy, it is a formal portrait where he is wearing what were probably his best clothes. The style of cravat also indicates someone dressed for a formal occasion.
Also, with all apologies to South Africa, if the portrait was painted there, it is likely that older fashions were slower to die out and new fashions slower to be adopted.
The other portrait looks to have the sitter in a more relaxed pose and in less formal attire. It is also possibly a pencil sketch rather than a painting?
Do you know anything more about the images? They don't appear to be framed, but presumably they were once?