There's a 67 rear seat speaker that has the number 18971. See pic from MPC.
It appears speakers can have different numbers than the 18808 listing.
I found what you referenced. I think this is a prime example of what is commonly referred to as "creative part number engineering", as applied to and by Ford. Ford engineering groups would come up with a "new" part, so they gave it a "new", and in many cases, part number, complete with a different base number. It worked. They got it made, installed and the car sold, and left it up to the service department to "correct" the different numbers.
The topic title indicated a 1966 application, so 67 would not work. However, with my Dewalt Sawzall, I could get it to fit, but not for a 66, and not for a Shelby.
Jim