"If I remember correctly the early Colt 20 round magazines had a metal follower and the Colt marking on the base plate was different (either larger or a smaller "COLT" ). The finish used on the magazine body has changed over the years. The early ones had a finish that when worn and used was burnished yellowish gold."

I have something like a dozen 20 round M16 magazines, all of which are stamped on the baseplate as "Colt AR-15" and have the metal followers. Most have that goldish color, which I think is a chromate conversion coating to control aluminum corrosion.