my 67 has 2 holes in the radiator support for 2 horns, but only came with 1 horn. and a single wire to it. the other hole doesnt have a ring or a mark where another bolt or horn would have been in its past.
this was originally a stripper /6 car. i dunno if that means anything. mebbe the cars that had more options came with a dual hi note / low note horn setup, and the Elcheapo models had to make due with one horn.