Go back to Diesel Giant pictures.
You will see him grinding out the pin (or called button)

This pin is what holds in the ignition lock housing.

That is why you can only rotate the housing 15 or 20 degrees.

If you feel the housing with your hand, near the steering column, on top of the ignition housing you will probably feel this little button (pin)

If it isn't depressed or ground out you will not be able to remove the ignition lock housing.