Agree with Frank's post above. Did find an interesting option listed at Novaresource.org. RPO U03 is a low note horn installed on 887 cars in 1965 and 479 in 1966. No numbers for 1967, which doesn't necessarily mean there weren't any, just maybe no numbers available.
A couple of dealer installs available for '66.....a tri-volume regular style horn and a matched horn I assume you could add to the one on your 100 series. In '65, there was the trumpet style tri-volume available.
Nothing to do with Chevy IIs, but my buddies 2019 Mustang Bullitt had only one horn. So, it's a 480 HP muscle car with a horn that sounded like a '48 Austin. Apparently, a Ford cost saving measure in 2019

. He remedied the situation by putting in 3 horns from a 25 year old Cadillac. Sounds like a freight train now.
Bob