This accessory set is also called a pillow block or curved block.
The block’s radius is defined for a specific tube diameter. Meade and Celestron SC telescopes, in particular, have pre-drilled holes with screws, which are designed to be used to attach a radius block. Using these, a dovetail bar can be attached to the tube to connect it to the mount. Since most telescopes are supplied already equipped with such a bar, the more common use for radius blocks is to install a second bar for a second optic or a guidescope.