If you mean the cylinder, that plunger prevents the cylinder from turning to the position where it can come out. There are notches inside the body that hold it in place until turned past the plunger.
As far as mounting the switch, there's a bracket that attaches to the bottom of the dashboard and has screws that engage the ears you can see in the first picture I posted. Here's a pic of the bracket.
Reproduction brackets, switches, and key cylinders are available from
Classic Parts and other vendors.
