Actually, there is no "other" cup inside the body. The separate cup is only in the limb itself. It needs to fit through the hole at the top of the limb, yet be large enough that it won't slide out easily.

I really need to make a detailed CAD-type drawing of the joint in cross-section to accompany the page on restringing. In this case, a picture is worth waaaay more than a thousand words.

Taffy