I remember seeing the first ones around the late 1980's. Not sure about the exact first time, but that doesn't matter in your case. The first ones I saw had really nice blueing finish rather than parkerizing which prompted me to look at them. I remember cocking back the op rod and it fell out too easy at the notch. Hmmm, 1989?
They came with bayonet lugs too. Century Arms stamp might be on it also.