If your looking for a horn, are you 6 or 12 volt? if 12 volt there are plenty on Ebay.
6V are rarer and thus more expensive, it depends if you want the sound of a Rist/Klaxon, or the appearance, if the latter, then Willies are the bee's knee's, if it's the sound your after then hunting about on Ebay, or contacting our cherished suppliers for an original Rist, Klaxon, or similar is the way - I've had a couple of excellent (and to me, sensibly priced) similar 1920's 6v horns from one of our cherished suppliers, just a phone call (or two) away.
Most people just say Chummy and give the year, the spec changed so much in the 1920's
The AD type appears to refer to the body tub, which was used from 1926 to 1930. During that period Austin made other changes, changing the engine from Mag to Coil, having a chrome radiator, and moving the lights. The chrome rad and lights on the radiator on this car say it is a 1930 Model Year style car. So would have a coil engine.
Location: 3103 West Washington Ave Yakima, WA 98903