choak

Auto Interchange Systems' 1950-65 General Motors Interchange Manual lists part 9000514 for the high note, and part 9000513 for the low note, for all full size Chevrolet cars from 1955 to 1965; and numerous other (smaller) Chevys, as well as Buicks, Oldsmobiles, Cadillacs, and the Pontiac Tempest depending on year. Trucks are not listed. If your one horn matches one of the part numbers, the other part number should be your missing horn. If not I might suggest you see if your Chevrolet dealer has an older Master Parts Catalog. Another option would be check the part numbers in a non-Cameo 1955 Chevrolet pickup. I doubt the horns in the Cameos are any difference then the horns in the 1/2 to 1-ton trucks.

If you post the model number and serial number I may be able to help pinpoint the year. The Herman Sass book, REO TRUCKS 1906-1967, has some model year serial number listings. Also a good frontal picture of the truck may help. Lastly, is the truck's Ability Rating plate still on the one of the door sills. A good picture of the Ability Rating plate may also help.