on my PHS , it states
Exterior decor group 324 $57.93
(I`m skipping engine , transmission)
Radio Push button 382 $61.09
Horns - dual 414 $4.21
Belts de Luxe 431 $36.86
Console 472 $ $53.71
PS var ratio 501 $105.32
Glass soft ray W/S only 532 $22.12
Custom trim option 554 $78.99
Head restraints 571 $16.85

and mine is a 69 convertible...
looks to me like dual horns were an option

mine was built in oct 68 also in Lordstown...last digets U107294, maybe close to yours?

my car only has ONE orig horn left in it though...I disconnected it ,addedv two air horns