Aim Sports ARDCA Dust Cover Mil-Spec AR-15, M4 Black 3.10"
https://www.tandktactical.com/shop/815879018830-aim-sports-ardca-dust-cover-mil-spec-ar-15-m4-black-3-10-8068
https://www.tandktactical.com/web/image/product.template/8068/image_1920?unique=761bde8
Aim Sports Dust Cover Assembly Mil-Spec AR15/M4 ejection port. Includes cover, spring, rod and C-clip.
