jss1956's horn is the correct single horn used on 47-52 trucks.

healingbear's horn was used as single horn in 1953-1955st; and, was one of the accessory dual-horns sold by GM from 47-55.

I think that a horn relay was not used with single horns until 1953; but, a horn relay came with the dual horn set (high-note and low-note clamshell horns) in earlier years.