Final Notes:

Horn dispersion does appear to be roughly constant directivity up to where the driver exit angle/horn throat transition causes diffraction & other irregularities. This horn is fairly large so the horizontal pattern control extends well below 1000 Hz. Vertical begins to widen below ~2000 Hz. Rated dispersion pattern is close to what I measured, though I measured bit wider in the vertical coverage perhaps closer to 50-60 degrees instead of 40.