PTS Low Profile QD Sling Swivel - S
The D loop on type S is moved a bit downward for providing more space and slimmer size while you need to attach with a limited space.
The D loop on type S for a hooktype attachment and can withstand strong pulling forces in all directions. 
Made of a strong steel alloy, the swivel has been tested to withstand up to 300kg of load.
The QD sling swivel also features a recessed button, a design feature first seen on the PTS ESP-M2, which prevents accidental activation.