I don't know what it's called or the part number but it's a simple black plastic support that fits onto the rail and looks like a "T". The height can be adjusted as needed and you can place it anywhere along the rail.
Christoph Greiner, who is my Arca dealer, sent me one with the bellows. You can ask him, he should know. He's extreemely helpful and knows nearly everything about Arca Swiss cameras.
His internet adress is: http://www.greiner-photo.de
. He's pretty quick to answer emails.