"the International Society for the Suppression of Savage Customs"
Flag of International African Association (as well as the Congo Free State and the Belgian Congo)
Public DomainFlag of the Association Internationale (as well as the Congo Free State and the Belgian Congo) - Credit: Wikimedia Commons

Conrad may be making a sarcastic allusion to the Association Internationale pour l’Exploration et la Civilization en Afrique (International Association for the Exploration and Civilizing of Africa), the front company used by Leopold to gain his foothold in the Congo. The Belgian monarch employed the Association as a smokescreen, disguising his colonial venture as a philanthropic mission dedicated to civilizing Africans.