Ford 9" rear ends have an option for a fill and/or drain bung to be installed on the housing. The Ford 9" center section has a bung on the driver's side of the center section that can be used to fill the rear end with oil. On certain applications like the 1982-2002 F-Body Camaro and Firebird this can be very difficult to access due to the torque arm mount. For any application there is no easy way to drain the fluid.
Having the fill and drain bungs welded on to the housing makes for hassle free filling and draining of fluids and we highly recommend going with this option.