Sorry - I thought Marrussia used it too...
I know I read that the HRT was using a whole Williams rear end, including gearbox.It was described as a current gearbox somewhere. i assumed williams would have been keen to have 4 cars testing bits and allowed it.
perhaps someone can identify it:
TBH I didn't like the Williams rear end when I first saw it(it just look shrink wrapped around the mechanicals with the sharp transitions that have been mentioned by flyboy)but last years was much smoother and had decent pace.
I dunno...maybe, like the walrus nose - stop flogging a dead horse and fill the area with body work as has been suggested.