M Aerodynamics silencers are different, too (m)
In addition to the hidden/exposed exhaust tips, the rear bumper skin is different on the wagon, so perhaps that is the reason that all silencers are different between sedans and wagons, even the ones for M Aerodynamics:
Sedan -- 18101407970 effective 2/2001
Wagon -- 18107832050 effective 3/2001
All of the aftermarket systems I've seen specify different silencers for wagons, too.