For what it's worth, here's what I found on the knz.org website:
"When the British government annexed Zululand in 1887 and established several magisterial districts, it was decided to administer that of Mthonjaneni from a town named after the resident commissioner - Sir Melmoth
Osborn."
Clearly, good old Sir Melmoth must have got his name from somewhere ... sounds vaguely Scottish ... but it might give you a lead.
As with
Andy's sensible suggestion, the spelling isn't an exact match for what you're looking for. But Melmoth is pronounced the same way as Melmuth would be, so it would be an easy transcription error, just as the capital letters H and M can look very similar in some handwritings and end up being confused as a result.