Nicolas, my understanding is that the design of the brake booster was changed in 2002 to incorporate a bellows on the booster to keep out water. Consequently, the old style booster was no longer available from that point on. In order for the new booster to work properly, there were other parts that need to be changed. Here's the link to the TSB:
http://www.pelicanparts.com/techarticles/Boxster_Tech/57-BRAKES-Booster-ABS/TechBulletin.pdf