This is a Burris Mount known as the AR-Pivot Ring 30mm, model 420168.
Quickly and easily mount the AR-Tripler inline behind non-magnified optics. For close quarter shooting" flip it to the side with the push of a button. To engage targets at longer distances" flip the AR-Tripler to its upright position and it automatically locks into place.
Features:
Protected by Burris Forever Warranty
QD mount makes installation a snap