There were some changes to the E46 suspension in 2001. According to RealOEM.com, in April 2001 there was a change to the rear shock, for example. I don't know what exactly changed, but I would guess the shocks are valved differently. There may be other changes, go to RealOEM.com, plug in your models and it will show you the break point for any changes ("From" and "Up To" in the part number diagrams).
BMW CCA member - Peachtree Chapter