They used a trumpet style horn on Chevy/GMC trucks from '47-'52 mounted on the intake manifold. The ear sticking up on the manifold between the carb and the fan with the two holes in it is the "perch" for the original horn.
[img]http://ebay2.ipixmedia.com/abc/M28/_EBAY_93a4d72e289754fe6a69ff48d9786d3a/i-1.JPG[/img]

They went to a seashell type horn in '53 still mounted to the manifold and then in '54 they offered an optional pair of seashell type horns with two notes; high/low for a combined sound.