The p/n for the Coupe seat frame from the dealer is A 123 910 05 22. Searching EPC net defaults to the old p/n of 116 910 20 22 and doesn't provide the cross reference. The 123 p/n shows compatibility for 280CE and 300CD.

The dealer in San Antonio checked availability and shows 100 of the frames in Germany. Of course, you'll need to pay $414 if you want to go this route.