The original 1953 horn was supposedly the seashell-style horn (first year) and was mounted on the "boss" on the intake manifold, forward of the carburetor (as stated by ddalesr3).

The optional dual seashell-style horns were mounted on the firewall (or other locations) in 1953 and earlier Advance-Design trucks. In 1954, the standard seashell horn (or optional dual seashell-style dual horns) were mounted on the front side of the radiator mounting frame (there ware factory-installed captive nuts for two horns on the 1954 radiator frame).

Tim