I have found a knife in Manfred A. Zirngibl's book ' Rare African Short Weapons' (1983) that resembles Colin's knife.
The author shows a picture of a knife with 'forked blade'. This piece is made out of copper.
This is what he writes : "The design of the two knives depicted here is sufficient to classify them as cult or currency knives. They were manufactured and used by the Yoruba. Both the knife on the left and the spearhead-shaped one with the strap on the handle were made of brass-like alloys. Surprisingly, the embellishment of both knives is almost identical.
The author was able to obtain the knife on the left at an out-of-the-way market on the border between Togo and Benin in 1979. The old merchant seemed to know its exact use and tried to explain it as 'money for voodoo'. Unfortunately, it was not possible to verify this."