On some rear ends, especially older ones, the part that holds the gears is a separate peice from the axle assembly. It bolts in from the front of the housing. the ford 9 inch differential is one of the more common rearends of this style that people are familiar with. Nice thing about the 3rd member rears is that the entire gear carrier can be removed from the axle housing without removing the housing from the vehicle. Makes for much easier service.
Here's a pic of mine. The cleaned off spot on the right side is where the code number is stamped.
