I missed the rivet.

Yep it is a ball bearing cage. In this case, STOP DRIVING NOW. The cage keeps the balls evenly spaced around the bearing, when the balls all end up on one side, the races move off center allowing balls to fall out. Once this occurs the gears barely mesh / balls fall between the gears. The case can split / wheels lock solid if metal gets between the gears. STOP DRIVING NOW, IT WILL GO BOOM ! ! ! !