It's the original locking pin. It locks the foreend-barrel assembly in the correct position when the barrel is screwed into the receiver. A "Deeley"-type latch retracts it into the foreend to allow the barrel to unscrew from the receiver. Someone later added an additional clamping screw in the left side of the receiver ring to take up any looseness that developed over the years. Follow the link http://forums.nitroexpress.com/showflat.php?Cat=0&Number=316604&an=0&page=0#Post316604 posted above and scroll down for the pics.