Ignition Lock

On my '74 Valiant, the chrome piece that the key fits into the column will do a 360 degree turn with no key in the ignition. It won't start the car, (unless you insert the key) but it spins all the way around. I've seen this on some older Chrysler vehicles before, but have no idea what the problem is, other that I assume something is worn or broken.

Any idea what's wrong?