Steel D-link / D Shackle.
A Steel D- link / D Shackle, also known as a gyve, is a U-shaped piece of metal secured with a clevis pin or bolt across the opening, or a hinged metal loop secured with a quick-release locking pin mechanism. The term also applies to handcuffs and other similarly conceived restraint devices that function in a similar manner. Shackles are the primary connecting link in all manner of rigging systems, from boats and ships to industrial crane rigging, as they allow different rigging subsets to be connected or disconnected quickly. A shackle is also the similarly shaped piece of metal used with a locking mechanism in padlocks.
Gyve, D-shackles are narrow shackles shaped like a loop of chain, usually with a pin or threaded pin closure. D-shackles are very common and most other shackle types are a variation of the D-shackle. The small loop can take high loads primarily in line. Side and racking loads may twist or bend a D-shackle. The pin is threaded and one leg of the shackle is tapped.
The pin may be captive, which means it is mated to the shackle, usually with a wire. The threads may gall if over tightened or have been corroding in salty air, so a liberal coating of lanolin or a heavy grease is not out of place on any and all threads. A shackle key or metal marlin spike are useful tools for loosening a tight nut. This is done by looping mousing wire or a nylon zip tie through the hole in the pin and around the shackle body. For pins that have a cross-hole in the threaded end a cotter pin can be used. One disadvantage of wire is that mousing can introduce galvanic corrosion because of material differences; it is especially bad when used in places where the shackle is exposed to air and water.