According to my research of online retailers and other threads, the Bosch # is the one that cross-references the M/B #. Prices very widely up to $1100 on Pelican Parts for the OEM M/B part. This M/B # seems to also be one that is widely used on most M/B models.

Here's another ebay listing for a cheaper brand (with no p/n) and the 288 different models it's compatable with...including XFires!

http://www.ebay.com/itm/New-Smog-Air-Pump-Mercedes-Benz-E-Class-CLK-ML-SL-S-CL-C-G-CLS-SLK-R-E320/371088826219?_trksid=p2047675.c100010.m2109&_trkparms=aid%3D333008%26algo%3DRIC.MBE%26ao%3D1%26asc%3D20131231133846%26meid%3D5bc806939c544b018aa1a58ce42b65f2%26pid%3D100010%26prg%3D20131231133846%26rk%3D1%26rkt%3D24%26sd%3D380843484252