Fold over slaps were used for many vintage mass produced pieces.
Because of the circular design and the relatively weak area where the inner portion of the clasp retracts into the hollow circle, spring ring clasps are not as secure as some other options.
The trigger opens the clasp, allowing you to attach the other end of the chain or necklace.
You'll find these on fine jewelry as well as on handmade items.
These closures are typically ovals, but they can be other shapes as well.
To operate the clasp, you pull back a tiny metal trigger.