A hi-note and lo-note pair of snail-like horns were GM accessories for Advance-Design trucks (at least from 1949-1955-1st, maybe back to 1947). Some people mount the pair (up through 1953) on the intake manifold or on the firewall or on the inner fender (I think there are earlier discussions of this accessory dual-horns, with links to pictures).
In 1954/55, the dual horns were mounted on the grill side of the radiator housing/frame. In these years, the standard horn (lo-note) was snail-shaped and one hi-note horn was available as an accessory.
