Hi,
I have a '66, and have yet to figure out the horn. I can only assume after some research that there were two types of horn activation at the steering wheel - once mechanical and one electrical. My '66 has a mechanical activation, and I ended up with an electrical activation horn button. If you are having the same situation on your '66, when you install the adapter the electical wiper on the 'bottom' of the adapter will close the horn circuit (and you will have an opportunity to hear how loud your horn is). If you remove the two screws and the brass wiper, you can use the adapter but the horn won't work. If indeed this is what is happening to you, I will be very interested in your solution. If it does turn out to be a mechanical activator hub adapter, I would be interested in the part number (and a source).
Regards -- Roger