This is a high-quality CNC manufacture extension tube from GSO.  Unlike cheap products, the mechanical tolerance of gap is precisely controlled to make any eyepieces and equipment fitted perfectly.

The extension is designed for any 2" eyepieces, and its inner a brass compression ring is designed to clamps equipment tightly but avoid any scratch on it.  The inside surface of this tube is black-matted helps to enhance imaging contrast and absorb any disturbing light.