There is a good article on Model A Klaxon Horns in the December 2012 Skinned knuckles magazine (Vol 37 - No. 5). It covers overhauling and tuning up the Klaxon "Ahh-Ooo-Gha" horns, like the ones on our 28-29 Plymouths. One interesting recommendation was to apply a film of grease to the face of the agitator to improve the sound. I tried it and it works well.