The Baader Hyperion 2" Fine-Tuning Stop Ring has a brass clamping ring with two thumbscrews.  It is designed to keep the 1.25" barrel of a Hyperion eyepiece that has been lengthened with fine-tuning rings from touching the mirror or prism inside your 2" diagonal, and possibly scratching or marring the surface. Additionally, you could use the HYP-STOP as a parfocalizing ring with any 2" barreled accessory.