The main difference is just the name. Until '39 they were simply called accessory horns. I have mine connected to a three position (on circuit one/off/on circuit two) toggle switch under the dash. In one position the single city horn blows & in the other the accessory horns blow. The center position neither blow which makes it handy when I'm messing with the horn button & don't want random honks.
You should have a heavy gauge full time hot wire from your battery to the center post on your relay. The horns connect to the passenger side terminal & driver's side terminal connects to the horn button, in my case through the toggle switch.
