Quicklink, also known as a Maillon Rapide from Peguet out of France.
Its origin of use, hence the name, comes from connecting two chain together. Which is faster if you screw a link in between them then forging and hammering a connection between two chain.
These days the quicklink comes in all kind of shapes and is still a favorite among climbers for its secure connection.
Square shape: the finest connector when connecting two flat webbing of same width.
Used as a connecting element for the A100 Viking harness when there is a chest ascender to be fitted.