These rods or rolls of brass and copper wire were known as mitakos, introduced in the Congo Free State as the primary form of currency for Leopold's new subjects:
The brass wire is cut into lengths called mitakos, this form of currency having been introduced by the late Sir H.M. Stanley. The length of the mitako, and so its value, varies in different parts of the country. At present there seems to be no limit to the amount of wire cut into mitakos, but as the natives use great quantities to make brass rings for the arms and legs of both sexes, it is difficult to say to what extent the currency is being debased… The natives pay no taxes in money or its equivalent, but instead are compelled to do this 40 hours' work per month for the State.
(Marcus R. P. Dorman, A Journal of a Tour of the Congo Free State)
The last detail is a telling one: the existence of meaningful currency might have undermined the forced labour upon which the state depended. Dorman also describes how African steamer crews would be paid in rolls of mitako, which became longer the higher they went up the river, an arrangement introduced by Stanley to encourage the Congolese crews to remain onboard until no longer needed.