A reversing contactor is simply two contactors interlocked. One contactor runs the motor in the forward direction while the other runs the motor in the reverse direction. The contactors are mechanically and sometimes electrically interlocked.
Ordering
For ordering, first you need two identical contactors. For example, two MC-6a contactors. Secondly, you would order a wire kit. For that same contactor combination, you would require a UW-18 kit. Each kit has a line and load side jumper bar. Lastly, you would select a UR-02 interlock unit.
Assembly
Assembly is straightforward. Any auxiliaries on the "inside" of the contactor should be removed to allow installation of the interlock. After snapping together the two contactors the wire kits can be installed.
Note that the wire kit marked "R S T" goes on the top while "U V W" goes on the bottom.