This unusual object is a pulley used for deep wells in the Sahara Desert. Saharan wells, up to 250 meters deep require a very long rope and a large bucket. The bucket, full of water, can only be lifted by a mule, the rope running over this pulley. Over time the friction of the rope on the pulley creates grooves. When the grooves get too deep the pulley has to be replaced, and is discarded. Each pulley is unique in shape and wear and is one of a kind. Although not tribally associated - since wells are used by everyone - the people who use them most would be the Tuareg and Fulani. Please see the photos here that I took in the Sahara desert in Niger of pulleys like this one in use.

This pulley measures: 5" long x 4 3/4" diameter (13 x 12 cm)