I'm not sure, but I had the same problem with the aluminum insert when I ordered an OE pad through PartsShop. Apparently that's all that MB makes any more.
But, Phil at PartsShop worked with me to return that one and get an aftermarket one that didn't have the aluminum (cheaper too). The quality seems very good and no problems after probably a year -- no sagging or flaking or anything.
I used 3M Super Trim adhesive to attach it. Also I didn't worry about getting all the old adhesive off, just scraped off the big chunks.
1999 E320 Wagon