A stamp booklet is/was primarily sold in the Kruger Park for airmail postage on postcards being sent out of the country. Because of that these stamps are rarely seen used in South Africa. A sheetlet was also printed with variations on the labels.
Thanks to Jan Korûbel for the short-term loan of the covers that have the variations.
Here's the booklet pane that's available in Kruger:
Here's part of the sheetlet on cover with the variations on the label:
Here's another label variation:
And another label variation: