This is an adapter for M42 and T2 lenses. It snaps into the undercut that every (metal) screw mount has. This specific adapter is for undercuts with ~1.2mm height. Older lenses (pre 1985) might exhibit larger undercuts (with the effect of the lens wobbling. Use https://clara.io/view/8f7fa50c-ff12-4c75-96ee-22afded3b298 for those.