In reality, the hallmark of Leopold’s brutal regime was not so much severed heads as severed hands.
Failure to meet rubber collection quotas was often punishable by death; in order to prove they had killed someone rather than used their munitions for hunting, African Force Publique soldiers were required to provide a hand for each bullet used. In practice, soldiers sometimes 'cheated' by simply cutting off a hand and leaving the victim to live or die. Sometimes the hands were collected by the soldiers of the Force Publique, sometimes by the villages. There were even small wars where villages attacked neighbouring villages to gather hands, since their rubber quotas were too unrealistic to fill. As a consequence, the rubber quotas were partly met in dismembered hands:
The baskets of severed hands, set down at the feet of the European post commanders, became the symbol of the Congo Free State. ... The collection of hands became an end in itself. Force Publique soldiers brought them to the stations in place of rubber; they even went out to harvest them instead of rubber... They became a sort of currency. They came to be used to make up for shortfalls in rubber quotas, to replace... the people who were demanded for the forced labour gangs; and the Force Publique soldiers were paid their bonuses on the basis of how many hands they collected.
(Peter Forbath, The River Congo: The Discovery, Exploration and Exploitation of the World's Most Dramatic Rivers, 1977)