Like another poster said, the key was common to many thousands of tractors. It is a short key with a rib formed by bending or pressing the steel. It is not a BS key. If you do have a BS key, then someone has changed the ignition switch. The original ignition switch on the 300 is quite stout, and should last nearly forever. I'm quite sure that your Case IH dealer has one available, but any tractor bone yard will have hundreds of those keys.
Paul in MN