The 55-1st Chevrolet truck is an A-D (Advance-Design) truck.
The
horn contact spring and button was/is the same for many years (47-66).
As truckernix stated, there is one wire running through the steering column to the horn relay (and then to the horn). The horn button completes the electrical circuit when the
grounded contact plate under the horn button touches the horn contact button (the plate in the photo is a 54/55-1st plate).
