Metal Styling Tack/Shawl Pin
To attach to knitwear unscrew and remove the back disk. Work the threaded cylinder with the front disk in between the stitches from front to back. Replace backing disk and screw together.
For semi-permanent attachment of this, and similar, closure(s), place a tiny drop of clear nail polish inside the threaded cylinder before screwing it in. This keeps the screw snug until unscrewed.