This is a rebreather safety mouthpiece (aka gag strap) manufactured by Drager and sourced from rEvo; their part number R521. This item features a generous sealing surface around the lips of your mouth and includes a replaceable silicone head strap and tie wrap. In addition to the rEvo DSV and rEvo BOV, the mouthpiece fits most other rebreathers and 3rd party BOV's such as the Shrimp and the Divesoft. We also offer the Apeks 2nd Stage Bungee Necklace which some people have adapted as a mouthpiece retaining strap but does not seal around the mouth.