sKiZo
Hates received: 92644 43.20°N 85.50°W
The item in question is the (late 70's or early 80's) AKG K240 Sextett LP (late production). The Sextett is apply named for the six passive radiators that surround each headphone driver. To my understanding the radiators are membranes that allow certain frequencies to pass through them and not others, resulting in a fuller frequency response.
Wonder if those might be kissing cousins of the Realistic Pro 50?
The passive radiators are vented in back, but not the center driver, apparently to minimize resistance and increase travel. Excellent sound, and exceptional comfort.