I can't add much but they were issued without gum on 6th August 1947 and were printed by Imprensa Nacional de Luanda.
"CTT" are the initials of the postal service at the time, "Correios, Telégrafos e Telefones" (the same as in Portugal) so maybe this was their logo or it's a logo for their airmail services.
The rough shield in the middle shows the arms of Portuguese West Africa (Angola) although you can't see much of the elephant or zebra on the right-hand side.
