Hi KD,
There is no Heil air transformer on the A and F. Both were full range "Walsh" drivers comprised of 3 separate sections made from paper, aluminum, and titanium. The A is 18" and the F is 12". Both consumed more horsepower than the output of any 60's muscle car save the L-88 option.
I know at least one manufacturer, RTR, that used the Heil air motion tweeter extensively in the 70's.
OK, this is really a separate topic and sorry to get off course.