OK, done a bit more research, I don't think it can be one of the 56 B9s. If we look at the headlights, the 56s have some of the lights blanked out, but the one in for repaint clearly has lights there.
On further inspection, it seems that there seems be similar damage on both the 'mystery' bus and 6050 that I couldn't see on any other possible vehicles. At least from photographs on Flickr! Although that could just be the angle of the cameras!
RE: Unknown B9 repaint