Here are two stamps that were part of provisional stamped postcards from Orange Free State.
Stamps were affixed to postcards and then overprinted with the seal shown to create temporary stamped postcards. The overprints are found in five different dies.

(I'm guessing this is Die 1.)
Here is a used example that appears to have been postmarked "Heilbron" in 1898.

Unfortunately, it has a pretty significant crease.
I don't have any examples of the complete cards yet. If you have one, feel free to add it.