Reaction Disk

You probably want something in the rubber hardness range, like tire hardness. Pretty vague, but I mean not jelloey and not super hard urethane. I think that the way it works is it distributes the movement from the pedal and booster to the master cylinder. As you push the brake pedal some of the movement is absorbed by the rubber disc. That's why when it's gone, you get the on-off feel. If you go too soft and jelloey, you might get a good feel until the rod bottoms out through the disc then you'll be back to off-on. If you go too hard you'll just get a less-pronounced off-on feel.

Just a guess. The weird names that Nissan gives parts have some clues in them though.