I've seen Japan spec 604-8KS ? with this style rear cover. Not sure why they would want a larger rear chamber unless it's to add more damping material, though FWIW, I had some pleasing results from replacing the rear cover of a pair of 802/8 ohm with a damped TL.

GM

edit: Didn't see the latest picture, so now I'm wondering if the 604s I saw had 'factory' rear covers.