Dear Duccio,
Thank you for posting the link to the interesting website
www.memoire-africaine.com. However, I am afraid there is some missunderstanding here. The knife you describe as "bottom left" is more probably from the northeastern part of Nigeria or from the northern Nigerian borderland. The shape of the handle is in Hausa style, the shape/style of the blade is like from the surroundings of north Cameroonian city of Maroua, the engarved ornaments on the blade are typical for Sahel. You are absolutely right, the handle decoration resembles the style of Kuba.
Best regards,
Martin