http://www.porsche.com/all/media/pdf/originalparts/usa/986_USA_KATALOG.pdf
Look on page 153. According to the Porsche original parts "USA_KATALOG.pdf," part # 993 362 138 01 is for stock, (2004) Boxster wheels in size 9 J x 18. You sure yours are 10"? Odd that they would list that part # for 9 J x 18" wheels if yours are 10"
! Unless they changed the part number to indicate a different wheel width between 1999 and 2004...