Found another piece of information
http://www.blackdiamondperformance.c...es_porsche.htm
It shows that the part no. of Boxster S rear disc is the same as that of 996 rear disc. May be dimensionally, they are the same. Stock units probably differ because of drilled (996) and non-drilled (Boxster S) while aftermarket performance units are usually slotted or drilled so they just use the same disc for both models. Just my guess.