This is the familiar grounding switch. The tube in the right end accepts a bullet wire connector and that part of the switch hits the threaded part which is screwed to the door jamb, grounding it. Only works on a steel car. Advantage is that is smaller and less conspicuous than the switch below.
This switch has two bullet plugs in the back and is insulated from the body. It can be used as a grounding switch on a 'glas body or as a hot switch on any car. Bring a full time hot wire to one terminal and a lead wire to a grounded dome light in the other terminal. This eliminates the need for the relay like Horvath used in his installation.
Both switches are available at any parts store.