Aluminum AR end plate with a QD sling socket, sold with a matching aluminum castle nut.

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Ascend Mod1 QD Endplate/Castle Nut Kit
The Ascend Mod1 QD Endplate adds a quick-detach sling socket to the part that already retains the rear takedown pin detent and spring, which puts a sling point at the receiver without an extra adapter. Next Level Armament sells it as a kit with an Ascend aluminum castle nut.
The plate is anodized aluminum, 0.2 inches thick, and weighs 0.39 ounces, and Next Level lists it for both AR-15 and AR-10 rifles. The Mod2 kit is the upgrade, with an angled QD socket and a castle nut that locks with a set screw instead of staking.
Add the Ascend Armory Mod1 QD End Plate and Castle Nut Kit (Next Level Armament) to your build and see how it enhances your platform.