I picked up an AR-18/AR-180 magazine several years ago. It is a 20 round magazine with a black plastic follower and the floorplate has the markings
A crosshair reticle is to the left of the "AR18" and is behind the words "ARMALITE INC"
There are also three rivets equally spaced down the front wall of the magazine, which prevent it from being adapted to fit and lock in the mag well of an AR-15. It's condition is excellent to near new (barely any wear on the feed lips, none on the follower and mag walls and floorplate appear to have little wear on them). What is the rough value on this magazine?