I flew the AAL
707s in both front seats during the sixties and seventies, and this is my recollection of the antenna issue. I think all the 123s were delivered with the HF
antenna, and the 720s never had them, since they were built as domestic airplanes from day one. American never flew the 123s overwater since they had no international routes in those days, and the fleet didn't have HF
radios installed. Boeing built the airplanes with HF
antennas because that was how they were designed. It wasn't an option.
In the last days of the 707, as they were being retired to the boneyards (and the 720 went first), a deal was struck with the Air Force to provide engines and vertical stabilizers from them for re-equipping KC
-135s, which needed HF
antennas. I think that some 123s still operational with the airline had the original vertical stabilizer replaced with 720 stabilizers so that the Air Force could have the HF
antennas. Hence the pictures above of N574 both with, and without, an antenna. It's possible that the antennas were removed from the 123s and they were reskinned, but a swap of the stabilizers makes more sense to me. I could be wrong, but this is how I remember it.
To further confuse the issue, some 720s wound up flying for foreign airlines after retirement from AAL
, and pictures can be found in the database of them sporting HF
antennas. There seems to have been a great deal of stabilizer swapping, depending on the need of the operator.