On A-D trucks, the horn relay was used with the dual accessory "matched" horns (seashell style 47-55, available all years). The horn relay might have been used in 1953 when the seashell-style single/stock horn was used (mounted on the intake manifold). The horn relay was certainly used on the 54/55 trucks (stock seashell horn mounted on the front of the radiator housing).
A horn relay was not used on 47-52 trucks with the "trumpet-style" stock horn (but people might have retro-fitted a relay).
The horn relay is listed here for use with matched (dual) horns from 34-54. 
