This part is located on rear left and rear right of my 2010 E350. The rear right (pictured below) has what looks like a "crack" on the right side, while of the left side of the car, this "crack" is opposite. I was unable to locate that part using a search on 785.233.137.301 (or 801) which is printed on the rubber.

http://www.flickr.com/photos/101848577@N03/9760983702/

Thanks for the info