[edesign45]

I'm not an expert on this but I do believe the rear shock towers are different (larger) on the 72-73 chassis. When using later aluminum trailing arms on a 69-71 911, a Bilstein style shock will bind/rub on the chassis do to the geometry of the shock angle. A more slender shock like a Koni Classic can solve this issue if you insist on using AL trailing arms. This is not an issue on a 72-73 chassis.