Before '39 they were simply called accessory horns. Try cleaning the contacts. I'm no expert but what happens is when you complete the circuit by pressing the horn button an electromagnet is powered up with pulls the diaphragm in one direction. At full travel a 'nub' on the diaphragm makes contact with a spot that breaks the circuit allowing the diaphragm to go the other way. This happen really fast and the back & forth is what makes the honk. If you're getting a single thump the nub isn't making good enough contact so the diaphragm is being held to one side instead of vibrating back and forth.

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