UIT rail

The UIT rail, also known as Anschutz rail, is a standard used for mounting slings and other gun accessories in competition shooting, and is essentially a T-slot track shaped aluminium extrusion profile accepting attachments in the form of T-slot nuts, or similar.
A similar design is called Freeland rail, but the dimensions of Anschütz (UIT) and Freeland rails differ. The Anschütz rail is wide and shallow, and the Freeland is narrower and deeper. Accessories made for one of the two types are not necessarily compatible with the other.
Dimensions.
Dimensions below are approximate.