Revolution (RSZ): Avid invented this seal make up in 2003 to offer the best of both worlds. There is a rubber seal on one side of the bearing and metal on the other. A rubber seal offers great dust/dirt/water protection but at the price of added friction. Where as metal offers almost no friction but will allow water and dust to get in over time once the grease has spread out. Since most cars have enclosed hubs and diffs, there is no reason to have the rubber seal inside the hub or diff since no dust, dirt, or water can get to it. But you still have the protection from the dirt not entering from the outside because it is sealed with the rubber seal.
This is a bearing kit by Avid RC for the HPI Venture