Wagon and Sedan bumpers are not the same. I believe the Wagon bumper is narrower. Also the side tabs, I believe the sedans have nuts tacked on, and the wagons are just a plate, as on sedans you bolt the bumper onto the quarter panel through the trunk, and on the wagon you go from outside.
That sounds right to me too. While they aren't technically the same you can interchange them with some work. At least that's the way it was always explained to me. IIRC the rear bumper on my wagon was originally (or at least previously) on a sedan.
When the only tool you have is a hammer every problem starts to look like a hippy.