Matt, I had seen those pics, but never looked carefully. Now that I have, I have a feeling it's the wood strip the mic's mounted to that's causing the reflection. Would you say it's about 4 inches from the tip of the mic to the wood strip? If so, that's our first notch at 1.7 kHz.
Do you have the wood strip when measuring the horns in free air? That's what I meant when I asked about your mic setup in the earlier post.
Also, that round protractor thingy could also be causing reflections, especially in the absence of other reflections. But it would be pretty delayed, so the cycle would be much larger. In other words, the teeth of the comb would be spread apart further. Someone suggested fiberglass. It really works. About 2" on the reflecting surfaces should do it.
You might try suspending the mic off a string or something and see if the response is smoother. I have been known to use chandeliers and a string to suspend mics in mid -air for measurements