Has anyone done this prop or better yet is it a found item?
To your original question as to the key being a found item, I suspect (with nothing to back it up) that the key was made but possibly based on a found item. My reason being that so much money and time was spent on the movie to get everything exact, why cut corners? Sure it is a small item but it gets some serious screen time. Nothing I've read about the making of the movie suggests Kubrick was too concerned about time or money being short.
Let the search and discussion continue. Either it is a found item or like I suggested, based on something found. But it would be great to nail a replica one way or the other.