Sorry, I've checked the file I keep for my car with all the receipts, articles, etc., and it's not there. I must have thrown it away. But I'm sure the cup holder mounted in the rear area of the wagon, so that the unfortunate passengers in the 3rd seat could have a spot to keep their beverage.
If your dealer can't find it on his computer screen, you can place a long distance call or try an e-mail to the Mercedes dealer in Long Beach, California. Since they apparently carry the cupholder in stock, they ought to know the part number...
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".