James,
I just checked out the pictures that you have provided, but they are not the original style that I am familiar with. The original kind I am referring to have horizontal round parts on top of the base. They are similar to the ones Scott has posted in the E-bay listing, but check out the link to the picture that I listed above for a decent picture of an original.
Also your air horns are in a different position than the ones I'm used to. The style I'm referring to are more to the drivers edge of the roof and further back. In this position, they are in the right place for the cable to go thru the mechanical valve plate on the curve of the roof and to be fastened approx 12 inches forward. Does your trucks have the factory valve plate on them? I've got one, but it is in storage somewhere. But to guess at it, it is about 3 inches by 5 inches and has either a slot or a hole for the cable to run thru.
Now this all tells us 2 things. Either GMC and Chevy used different horns at different plants or your horns were possibly dealer installed or aftermarket installed. Of the 100's maybe 1000's of '60 to '66 and newer trucks I have seen with air horns, I would say about 95% of them have the style horns that I am referring to. It seems like GM used this style horn to about the mid 70's.
I'm going to dig out my accessory catalog to verify the correct brand name.
Bill
