The Vuka Alumina lineup isn't merely an entry-level version of Zipp's other bars. It's a re-think of their original system, it stands on its own, and has some very smart design features. There's more adjustment here than you see in any other clip-on bar system, yet Zipp keeps the hardware fairly minimal, easy to adjust, and aesthetically pleasing. The Vuka Alumina Clip is a chameleon. You can set it up to look like almost anything you want, creating almost any imaginable relationship between cups, extensions, and the base bar.
You can add stack to the bars via these custom risers, which raise both the extensions and arm cups together, preserving whatever relationship you established with them.