It's an IEC711 clone that's been pretty common among reviewers. The bad thing is that these clone couplers usually have a wide variance at their resonance peak 'intensity' and most reviewers tend to reach for an 8-9 kHz resonance peak to match crinacle's standard/methodology (or sort of). A good example for this variance is surfing through Etymotic models and famous models on
https://squig.link and comparing to different databases (linked at the bottom)
He posted a graph with a bit deeper insertion on discord and the peak moves to 10k as expected changing the overall upper-treble response quite a bit.
But I hope he can chime in more in-depth.