This is the Black Gates V-belt, not the clear round NW belt.
The V-belt is a better belt than the clear O-belt. But it only fits the NW 235, 240, 250, 260, 265, and early 270's. NW switched away from it to the round belt. The V-belt is far better because it almost never wears out and maintains full pump pressure for the driest possible pulp.

Any NW can be converted to the V-belt by upgrading the lower pulley to one with a 'V-shaped profile install of the square flat bottomed profile.