This
Tang Band W5-1611SAF driver is a dead ringer, right down to the "BC 120M" stamped in the rubber at the edge of the surround. Tho I wouldn't be surprised if it's customized for the Sierras.
I do know whatever it is, it's a very good midrange driver and I think is almost as big a star of the show as the RAALs. I listen to a lot of rock and metal where there's a lot of energy in the midrange and my music has never sounded so good. I haven't had these towers a year yet so I have a teensy bit of that "new speaker" feeling still, lol.