I know it's an old thread and this is my first post, but I thought those that replied to this thread might enjoy my find.
A few months back I was wandering around the York gun show in PA. Out of the corner of my eye I spotted a magazine in a dumpster of a display that had the AR-18 mag catch. I asked the guy to pull out the mag and found it was steel and parkerized. It also had the correct black AR-18 cut follower. However, the floor plate had no markings. Thinking I had a Sterling mag that someone swapped floor plates out on, I offered the guy ten bucks and he took it.
My memory failed me in that I forgot the Sterling mags had the hole punched in the side which this mag did not. I stripped and reparked the mag. I decided to do a little search on the interwebs when I came across this thread. Here are pics of my find - I think we now have two of these mags accounted for.Attachment 56071Attachment 56072Attachment 56073Attachment 56074Attachment 56075Attachment 56076