This article discusses availability of the Duo Dock top cover upgrade.

The Duo Dock Top Cover Upgrade (M1814Z/A) which is necessary to put a color duo into an original Duo Dock is no longer available.

However, the service part (922-0519, Top Cover, Duo Dock II w/door) is still available. So, if you need the taller top for your original Duo Dock, you will need to ask your authorized Apple Service provider to order this service part instead of the finished goods part.