That’s a very late (circa 07) AICS one year only extended spigot shown with standard spigot below:
View attachment 8221574
To remove it, remove the barreled action from the chassis and remove the skins as necessary.
Drive out the roll pin shown above. Drop the bipod stud. With a flat blade screwdriver, push the spigot as far forward as it will go. Wiggle out the sling hook- it’ll be stuck in there pretty well because a rubber puck is holding it in place, then push the spigot out of the aluminum frame.
Duck and cover because this isn’t an AICS thread…. It’s an Accuracy International rifle thread.