I’m using the MT-1 Accessory Tube Lens and MT-1/MT-2 C-Mount Adapter. Two thin brass rings came with the adapter. How are they used?

You can use the spacer rings to compensate for little irregularities of the position of the intermediate image plane on the MT-1/MT-2 C-Mount Adapter. They are extremely useful when trying to properly align a setup as the can easily correct for slight misalignment anywhere in the system.