Wow, standard, low and high horns. Thanks for checking. And thanks for the offer, but I scored some "N" bolts off a board member.
Someday I'll have to ID my 72. I do I see two horns however, one mounted below the headlight, and one mounted using the fender to core support bracket that's shaped like a trumpet. Appears original, the truck has an aux. battery tray.
I may have to rethink the "J" being a low note horn. Not much info. out there and I can't find a "J" horn. Low must be "L" and high must be "H" seems to be an over-simplification across vehicles anyway. More than a few notes in the "sea shell" form.
http://www.delcoremyhistory.com/Products/horns.htm