To add to the confusion, I'll invoke Ford "numbers".
The original horns specified in the 1968 Mustang Electrical Assembly Manual (AM0023) are:
C7ZF-13802-E for the STANDARD low horn (ref pg
;
C7ZF-13802-D for the "Air Conditioning" application (ref pg 76) which is what you are looking for; note the different "dash" letters;
This is where it gets "messy".
The 1968 Ford Car Parts (aka the infamous MPC) uses a "kit" for service replacement horn (B8AZ-13833-B - note the "B" prefix which is a 1950 era designed Ford part, and the base number change from "13802" to "13833), which specifies a lot (yes boys and girls, it IS two words) of different "brackets" (read adapters) and a standard "full size Ford" horn with a different base number - C7AF-13A803-A, which includes a "support" (not specified).
In other words, use the original and don't consider the Ford service replacement.
Jim