Undercut, means that the locating dogs are machined at an angle so that when the dogs mesh together they actually try to pull the gears together. this helps the gears locate faster and aslo resists jumping.

Errrr, a picture would probably be better, but I'm crap at drawing. Andy's a bit of a Picasso so I'll leave it to him. I'm putting an F3 box in my 300, so I suppose I could take a pic of that and a comparison shot of the NSR box. Don't hold your breath though 'cos I've just left the workshop and I ain't going back into that dark room 'til dawn.