I've found another subtle but "challenging" difference between genuine and copies, in the diaphragm. 
The edge of the main diaphragm is tucked into a slot in the "lid". The bottom half of the case has a fairly sharp edge that presses down into this slot to seal that edge very firmly.
BUT the shape (cross-section) of the edge strip is subtly different; the copy is pretty much square (about 2mm each way) but the OEM is tapered down into the slot, overall it has maybe 25% less material there.
This taper makes it easier to compress and seal; the copy is much more difficult to compress and makes it very difficult to snap the case shut.
I tried fitting a copy diaphragm into the OEM case and just couldn't get it closed at all! In the end I worked carefully around the edge of the copy diaphragm with a craft knife and pared off a sliver of material. Probably a bit too much as the diaphragm didn't then fit securely. But it enabled me to close the case properly and it tests as airtight.
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2003 D2 FL S8. Irish Green Pearl/Beige. Solar sunroof, auto-dim mirrors, electric rear seat functions, ski hatch retrofit; extended leather. Aftermarket DVB-T, reversing camera and full XCarlink (Bluetooth etc.).
2016 Volvo V40 T5 Cross Country (4WD) with ALL the toys including adaptive cruise etc. etc. Osmium Grey with Blonde/Charcoal leather interior. Polestar performance "optimisation".  (A much rarer model than a D2 S8 by the way!) Oh, and a brand new engine at just under 30,000 miles on the factory one! 
Finally: gone, but not forgotten.....
1998 D2 PF S8. AgateGrey/Platinum. Every option (I think) except electric rear seats, tiptronic steering wheel, ski hatch, towbar & dimming door mirrors.
e.g. Cruise control, NavPlus/TV, Bose, GSM, Xenons, Solar roof, Parking sensors, Alcantara/leather everywhere of course.  (internal dimming mirror added later)
1998 (very early) Ford Focus 1.8 Zetec; ABS/TCS, Heated screen/mirrors, Aircon, Auto-dim mirror, Leather, Trip computer, Cruise control, OEM Ford SatNav with CD changer.
And before that a lot of Rover 800s, a few oddities, a lovely Triumph Dolomite 1850HL with Overdrive and way back in my schooldays an Austin Seven aka Mini 850!
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