The thing that's holding them up is the shorting ring. Apparently, the first prototype reduced distortion by the amount required, but not down at as low frequency as required. Distortion improvement was only above 150Hz, and for a subwoofer, that's not low enough. The requirements stated distortion reduction should be down to at least 50Hz. This requires a very large shorting ring, and that in turn requires different materials and placement which also necessitates different machining.