HI Mitchco,
Your 2384 horn is 'pretty larger then STX825, or PHF95HF horns, but UHF response is may be almost the same behaviors.
I only wonder is it possible than no one in JBL have not ever tried to measure the mentioned 2453SL driver with the mentioned STX825 horn, especially as there has been a speaker with such driver&horn combo, bit mentioned notch has not been empathized in the speakers data information.
regards
ivica