Fills the open gap between the thread and the metal behind it. Also has a "pullout" hole, if for some reason the adapter breaks somewhere, just use an fitting allen/hex key and pull it out. The hole has a diameter of 2mm

might fit on other adapters to, measurements:

inner cirle: 11mm outer cirlce: 15.5mm pull out ring: 2mm

Adapter link: https://www.softairwelt.de/MP7-Adapter-von-11mm-auf-14mm