A 6V '32 horn on 12 V ROARS! There is no "Ahoo" because the voltage blows it to full RPM instantly. Most people would be more likely to assume a terrorist bombing than horn, but it'll sureashell get their attention.
One of the Model A gurus says just running the lead through the primary circuit of a Model A-B type coil gives the right resistance to make it sound like a stocker. Model A pats suppliers also offer a purpose made resistor or reducer of some sort just for this.
Model A horns use same motor and many of the parts as a '32 Boattail, so there is lots of info and some parts availability. These things are WAY ovebuilt mechanically and usually don't need more than a good cleaning, oil where the owner's manual tells you to put it, and attention to be sure brushes have clean contact with armature. There is an adjustment screw right out the back so you can tune the noise to your preference.