The bearing located at the front axle chain tensioner. Part of the bearing is an eccentric lock that is hammered to the side so that it clamps onto the axle. It is then secured with a set screw.
(One reason this locking mechanism may come loose is poor lubrication of the universal joint sleeves in the center joint. Loose bearing cups also increase the likelihood of the axle being pushed back and forth during steering and simultaneous loading).