your best bet would be to find a South African specialist,
indeed a Cape specialist that dabbles in revenues,
I would think this is a very specialised field.
I can show you an "embossed revenue" from India
that at least illustrates how they were applied.
Usually if these cancels were printed they would
have 3 tiny cirles with dates eg: 3 then 4 then 1876 etc
giving the date 3rd april 1876
These Indian embossed only have one circle,
in this case "31"
They are just used to prevent the stamp being reused.
These are contracts between labourers and employers.