Allen's Fixed Eye Snap Shackle.
Schaefer's Snap Shackles are designed for maximum strength with smooth, jam-free operation. The enlarged throat and hinge area handles shear and twisting movements effectively. The smooth shackle arc prevents jamming against pad eyes or sail grommets to permit easy sail attachment and release. Snap Shackles are designed to allow quick release of loads when pin is pulled.
While this is a convenience for many applications involving spinnakers and sheets; there is a danger that misuse or misapplication could result in damage to equipment or personal injury. Snap Shackles are never to be used to hoist people aloft.