ok I was just roaming around online and found this http://me.a-spec.ca:6001/Rotary/13B/ExPlate.JPG and the page before it just says "Some dislike the high pitched exhaust note of the naturally aspirated rotary engine, so the Mazda engineers have conceived a insert for the exhaust port that dampens the sound by letting the exhaust out in gradual staged chambers."

this isn't something stock is it? I've never even heard of it before

Yep, that's right. The N/A rotor housings use a different exhaust sleeve compared to the turbo ones. The sleeve is held in via a pair of roll pins on either side of the sleeve at the edges of the rotor housing. They're interchangable but they're VERY difficult to deal with.

It is stock and leaving the N/A exhaust sleeves in an N/A motor is quite a good idea (trust me on this).

B