I found another 288-(16)K Freq graph here: MachMat mainpages.

288-103.jpg

Much smoother on the circular "Oris" tractrix horn. The black line shows the un-eqed measurement and shows the rising response up to 1KHz, The upper part looks a lot flatter though. Save for the reflections, the 511-E throat blobs may have something to do with this too.

Coen