I recently purchased this magazine for a S&W 457. I haven't actually used it yet, but seems dimensionally the same as a S&W marked magazine. Metal work seems a bit crude compared to a known S&W magazine, but this is also wearing a S&W marked floor plate. No markings other than witness numbers.

The stern warning, "THIS MAGAZINE MUST HAVE A YELLOW FOLLOWER" is a new one to me. I don't pretend to have seen a ton of third gen magazines, but could somebody explain all this to me, especially why anybody would engineer something like this if magazine technology had been successful for most of a century?