All horns (single and dual) had a relay from 1953 onwards. 1953 was the first year that a "low note" "seashell" horn was standard (mounted on the intake manifold).
The accessory "add-on" high note "seashell" horn was a 1953-55st accessory
Earlier (pre-1953) standard, single horns on trucks did not come with a relay.
The 1947-52 accessory "matched horns" were two "seashell" style horns (and this set came with a relay).
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