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 a far better part because it almost never wears out and it 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 instal of the square flat bottomed profile. Just Order the Performance Upgrade Kit 1, http://severalgardens.com/product/performance-upgrade-kit-1/.