As I'm running out of envelopes with unique advertising on the front, I've opted to use this one from out of town.
Humansdorp, just west of Port Elizabeth, was founded in 1849 by Johannes and Matthys Human, who were joint founders of the Dutch Reformed Church there, says Wikipedia.
This envelope was posted to the PE city treasurer by the Humansdorp Divisional Council. The town today is part of the Kouga local municipality and Cacadu District.
These envelopes date from 1961, which is 13 years after the National Party came to power. Despite it imposing all manner of apartheid laws, it seems at this time Chinese-owned shops were still allowed to operate in so-called white suburbs. I remember visiting my great aunt in Cambridge, East London, in the early 1960s and walking a couple of blocks from her house at No 1 Courtenay Street, to a Chinese-owned grocer shop. Don't you love this name though, Wing King. I wonder if they winked as you came in. No sign of them in today's phone book, though.
There are a good many "African" companies in the phone book, but African Explosives is not listed. Wikipedia tells us it is still very much alive, mainly supplying the mining industry. It was founded in 1896 at Modderfontein after gold was discovered on the Reef.
King's Pharmacy is another firm no longer listed in the phone book.