As you might have noticed, all of the triangular stamps are actually tax (revenue) stamps.
Maybe because they are easy to recognised, and they are different from the regular stamps.
Receiving nice stamps implies you have to pay extra :-)
I chose tax stamps of Cameroun, Dem. Republic of Congo, Mauretania, Mali, Gabon and Tchad.
This last set has a very nice set of prehistoric designs on stamps. Although, I think the rhinoceros or what it is supposed to be on the 0,50 F stamp is more like a modern design (looks like a childs design to me)